The app opened, and the assistant began scrolling through the feed. While the professors had been lecturing about post-apocalyptic culture, some of the 230 or so freshmen in the auditorium had been having a separate conversation about them on a social media site called Yik Yak. There were dozens of posts, most demeaning, many using crude, sexually explicit language and imagery.

After class, one of the professors, Margaret Crouch, sent off a flurry of emails – with screenshots of some of the worst messages attached – to various university officials, urging them to take some sort of action. “I have been defamed, my reputation besmirched. I have been sexually harassed and verbally abused,” she wrote to her union representative. “I am about ready to hire a lawyer.”

In the end, nothing much came of Crouch’s efforts, for a simple reason: Yik Yak is anonymous. There was no way for the school to know who was responsible for the posts.

Eastern Michigan is one of a number of universities whose campus has been roiled by offensive “yaks.” Since the app’s introduction a little more than a year ago, it has been used to issue threats of mass violence on more than a dozen college campuses, including the University of North Carolina, Michigan State University and Penn State. Racist, homophobic and misogynist “yaks” have generated controversy at many more, among them Clemson, Emory, Colgate and the University of Texas. At Kenyon College, a “yakker” proposed a gang rape at the school’s women’s center.

In much the same way that Facebook swept through the dorm rooms of America’s college students a decade ago, Yik Yak is now taking their smartphones by storm. Its enormous popularity on campuses has made it the most frequently downloaded anonymous social app in Apple’s App Store, easily surpassing competitors like Whisper and Secret. At times, it has been one of the store’s 10 most downloaded apps.

Like Facebook or Twitter, Yik Yak is a social media network, only without user profiles. It does not sort messages according to friends or followers but by geographic location or, in many cases, by university. Only posts within a 1.5-mile radius appear, making Yik Yak well suited to college campuses. Think of it as a virtual community bulletin board – or maybe a virtual bathroom wall at the student union. It has become the go-to social feed for college students across the country to commiserate about finals, to find a party or to crack a joke about a rival school.

Much of the chatter is harmless. Some of it is not.

“Yik Yak is the Wild West of anonymous social apps,” said Danielle Keats Citron, a law professor at University of Maryland and the author of “Hate Crimes in Cyberspace.” “It is being increasingly used by young people in a really intimidating and destructive way.”

Colleges are largely powerless to deal with the havoc Yik Yak is wreaking. The app’s privacy policy prevents schools from identifying users without a subpoena, court order or search warrant, or an emergency request from a law-enforcement official with a compelling claim of imminent harm. Schools can block access to Yik Yak on their Wi-Fi networks, but banning a popular social media network is controversial in its own right, arguably tantamount to curtailing freedom of speech. And as a practical matter, it doesn’t work anyway. Students can still use the app on their phones with their cell service.

Yik Yak was created in late 2013 by Tyler Droll and Brooks Buffington, fraternity brothers who had recently graduated from Furman University in South Carolina. Droll majored in information technology and Buffington in accounting. Both 24, they came up with the idea after realizing that there were only a handful of popular Twitter accounts at Furman, almost all belonging to prominent students, like athletes. With Yik Yak, they say, they hoped to create a more democratic social media network, one where users didn’t need a large number of followers or friends to have their posts read widely.

“We thought, ’Why can’t we level the playing field and connect everyone?’ ” said Droll, who withdrew from medical school a week before classes started to focus on the app.

“When we made this app, we really made it for the disenfranchised,” Buffington added.

Just as Mark Zuckerberg and his roommates introduced Facebook at Harvard, Buffington and Droll rolled out their new app at their alma mater, relying on frat brothers and other friends to get the word out.

Within a matter of months, Yik Yak was in use at 40 or so colleges in the south. Then came spring break. Some early adopters shared the app with college students from all over the country at gathering places like Daytona Beach and Panama City, Florida. “And we just exploded,” Buffington said.

Droll and Buffington started Yik Yak with a loan from Droll’s parents. (His parents also came up with the company’s name, which was inspired by the 1958 song, “Yakety Yak.”) In November, Yik Yak closed a $62 million round of financing led by one of Silicon Valley’s biggest venture capital firms, Sequoia Capital, valuing the company at hundreds of millions of dollars.

The Yik Yak app is free. Like many tech startups, the company, based in Atlanta, doesn’t generate any revenue. Attracting advertisers could pose a challenge, given the nature of some of the app’s content. For now, though, Droll and Buffington are focused on extending Yik Yak’s reach by expanding overseas and moving beyond the college market, much as Facebook did.

Yik Yak’s popularity among college students is part of a broader reaction against more traditional social media sites like Facebook, which can encourage public posturing at the expense of honesty and authenticity.

“Share your thoughts with people around you while keeping your privacy,” Yik Yak’s home page says. It is an attractive concept to a generation of smartphone users who grew up in an era of social media – and are thus inclined to share – but who have also been warned repeatedly about the permanence of their digital footprint.

In a sense, Yik Yak is a descendant of JuicyCampus, an anonymous online college message board that enjoyed a brief period of popularity several years ago. Matt Ivester, who founded JuicyCampus in 2007 and shut it in 2009 after it became a hotbed of gossip and cruelty, is skeptical of the claim that Yik Yak does much more than allow college students to say whatever they want, publicly and with impunity. “You can pretend that it is serving an important role on college campuses, but you can’t pretend that it’s not upsetting a lot of people and doing a lot of damage,” he said. “When I started JuicyCampus, cyberbullying wasn’t even a word in our vernacular. But these guys should know better.”

Yik Yak’s founders say the app’s overnight success left them unprepared for some of the problems that have arisen since its introduction. In response to complaints, they have made some changes to their product, for instance, adding filters to prevent full names from being posted. Certain keywords, like “Jewish,” or “bomb,” prompt this message: “Pump the breaks, this yak may contain threatening language. Now it’s probably nothing and you’re probably an awesome person but just know that Yik Yak and law enforcement take threats seriously. So you tell us, is this yak cool to post?”

In cases involving threats of mass violence, Yik Yak has cooperated with authorities. Most recently, in November, local police traced the source of a yak – “I’m gonna  1/8gun emoji 3/8 the school at 12:15 p.m. today” – to a dorm room at Michigan State University. The author, Matthew Mullen, a freshman, was arrested within two hours and pleaded guilty to making a false report or terrorist threat. He was spared jail time but sentenced to two years’ probation and ordered to pay $800 to cover costs connected to the investigation.

In the absence of a specific, actionable threat, though, Yik Yak zealously protects the identities of its users. The responsibility lies with the app’s various communities to police themselves by “upvoting” or “downvoting” posts. If a yak receives a score of negative 5, it is removed from the site. “Really, what it comes down to is that we try to empower the communities as much as we can,” Droll said.

When Yik Yak appeared, it quickly spread across high schools and middle schools, too, where the problems were even more rampant. After a rash of complaints last winter at a number of schools in Chicago, Droll and Buffington disabled the app throughout the city. They say they have since built virtual fences – or “geo-fences” – around about 90 percent of the nation’s high schools and middle schools. Unlike barring Yik Yak from a Wi-Fi network, which has proved ineffective in limiting its use, these fences actually make it impossible to open the app on school grounds. Droll and Buffington also changed Yik Yak’s age rating in the App Store from 12 and over to 17 and over.

Toward the end of last school year, almost every student at Phillips Exeter Academy in New Hampshire had the app on his or her phone and checked it constantly to read the anonymous attacks on fellow students, faculty members and deans.

“Please stop using Yik Yak immediately,” Arthur Cosgrove, the dean of residential life, wrote in an email to the student body. “Remove it from your phones. It is doing us no good.”

At Exeter’s request, the company built a geo-fence around the school, but it covered only a few buildings. Students continued using the app on other parts of the sprawling campus.

“We made the app for college kids, but we quickly realized it was getting into the hands of high schoolers, and high schoolers were not mature enough to use it,” Droll said.

The widespread abuse of Yik Yak on college campuses, though, suggests that the distinction may be artificial. Last spring, Jordan Seman, then a sophomore at Middlebury College, was scrolling through Yik Yak in the dining hall when she happened across a post comparing her to a “hippo” and making a sexual reference about her. “It’s so easy for anyone in any emotional state to post something, whether that person is drunk or depressed or wants to get revenge on someone,” she said. “And then there are no consequences.”

In this sense, the problem with Yik Yak is a familiar one. Anyone who has browsed the comments of an Internet post is familiar with the sorts of intolerant, impulsive rhetoric that the cover of anonymity tends to invite. But Yik Yak’s particular design can produce especially harmful consequences.

“It’s a problem with the Internet culture in general, but when you add this hyper-local dimension to it, it takes on a more disturbing dimension,” said Elias Aboujaoude, a Stanford psychiatrist and the author of “Virtually You.” “You don’t know where the aggression is coming from, but you know it’s very close to you.”

Jim Goetz, a partner at Sequoia Capital who recently joined Yik Yak’s board, said the app’s history of misuse was a concern when his firm considered investing in the company. But he said he was confident that Droll and Buffington were committed to ensuring more positive interactions on Yik Yak, and that over time, the constructive voices would overwhelm the destructive ones.

“It’s certainly a challenge to the company,” Goetz said. “It’s not going to go away in a couple of months.”

Seman wrote about her experience being harassed on Yik Yak in the school newspaper, The Middlebury Campus, prompting a schoolwide debate over what to do about the app. Unable to reach a consensus, the paper’s editorial board wrote two editorials, one urging a ban, the other arguing that the problem wasn’t Yik Yak but the larger issue of cyberbullying. (Middlebury has not taken any action.)

Similar debates have played out at other schools. At Clemson, a group of African-American students unsuccessfully lobbied the university to ban Yik Yak when some racially offensive posts appeared after a campus march to protest the grand jury decision’s not to indict a white police officer in the fatal shooting of an unarmed black teenager in Ferguson, Missouri. “We think that in the educational community, First Amendment rights are very important,” said Leon Wiles, the school’s chief diversity officer. “It’s just problematic because you have young people who use it with no sense of responsibility.”

During the fall, Maxwell Zoberman, the sophomore representative to the student government at Emory, started noticing a growing number of yaks singling out various ethnic groups for abuse. “Fave game to play while driving around Emory: not hit an Asian with a truck,” read one.

“Guys stop with all this hate. Let’s just be thankful we arn’t black,” read another.

After consulting the university’s code, Zoberman discovered that statements deemed derogatory to any particular group of people were not protected by the school’s open expression policy, and were in fact in violation of its discriminatory harassment rules. Just because the statements were made on an anonymous social-media site should not, in his mind, prevent Emory from acting to enforce its own policies. “It didn’t seem right that the school took one approach to hate speech in a physical medium and another one in a digital medium,” he said.

Zoberman drafted a resolution to have Yik Yak disabled on the school’s Wi-Fi network. He recognized that this would not stop his fellow students from using the app, but he nevertheless felt it was important for the school to take a stand.

After Zoberman formally proposed his resolution to the student government, someone promptly posted about it on Yik Yak. “The reaction was swift and harsh,” he said. “I seem to have redirected all of the fury of the anonymous forum. Yik Yak was just dominated with hateful and other aggressive posts specifically about me.” One compared him to Hitler.

A few colleges have taken the almost purely symbolic step of barring Yik Yak from their servers. John Brown University, a Christian college in Arkansas, did so after its Yik Yak feed was overrun with racist commentary during a march connected to the school’s World Awareness Week. Administrators at Utica College in upstate New York blocked the app in December in response to a growing number of sexually graphic posts aimed at the school’s transgender community.

In December, a group of 50 professors at Colgate University – which had experienced a rash of racist comments on the app earlier in the fall – tried a different approach, flooding the app with positive posts.

Generally speaking, though, the options are limited. A student who felt that he or she had been the target of an attack on Yik Yak could theoretically pursue defamation charges and subpoena the company to find out who had written the post. But it is a difficult situation to imagine, given the cost and murky legal issues involved. Schools will probably just stand back and hope that respect and civility prevail, that their communities really will learn to police themselves.

Yik Yak’s founders say their startup is just experiencing some growing pains. “It’s definitely still a learning process for us,” Buffington said, “and we’re definitely still learning how to make the community more constructive.”