CONCORD, N.H. » A graduate of an exclusive New England prep school was cleared of rape but convicted Friday of lesser sex offenses against a 15-year-old freshman girl in a case that exposed a tradition in which seniors competed to see how many younger students they could have sex with.

A jury of nine men and three women took eight hours to reach its verdict in the case against Owen Labrie, who was accused of forcing himself on the girl in a dark and noisy mechanical room at St. Paul’s School in Concord two days before he graduated last year.

Labrie, who was bound for Harvard and planned to take divinity classes before his arrest put everything on hold, could get as much as 11 years in prison at sentencing Oct. 29. The 19-year-old from Tunbridge, Vermont, will also have to register as a sex offender for the rest of his life.

He wept upon hearing the verdict, and then, as his lawyers conferred with the judge, sat alone at the defense table, shaking his head slightly and looking up at the ceiling. His mother sobbed. His accuser appeared stoic and huddled with members of her family in the courtroom.

“Owen’s future is forever changed,” defense attorney J.W. Carney said, adding that the sex convictions will be like “a brand, a tattoo” that he will bear for life.

The scandal cast a harsh light on the 159-year-old boarding school that has long been a training ground for America’s elite. Its alumni include Secretary of State John Kerry, former FBI Director Robert Mueller, “Doonesbury” creator Garry Trudeau, at least 13 U.S. ambassadors, three Pulitzer Prize winners, and sons of the Astor and Kennedy families. Students pay $53,810 a year in tuition, room and board.

Prosecutors said the rape was part of Senior Salute, which Labrie described to detectives as a competition in which graduating seniors tried to have sex with underclassmen and kept score on a wall behind a set of washing machines.

The young man was acquitted of the most serious charges against him — three counts of felony rape, each punishable by 10 to 20 years in prison. But he was found guilty of three counts of misdemeanor sexual assault, using a computer to lure a minor for sex, and child endangerment.

Essentially, the jury by its verdicts signaled it didn’t believe Labrie’s assertion that there was no intercourse, but it also didn’t believe the girl’s contention that it was against her will. In the end, it found Labrie guilty of having sex with an underage girl.

The girl is “leaving with her head held high,” said Laura Dunn, a spokeswoman for the teenager and her family. “It was a step in the right direction.

But the girl’s family lashed out at the prep school, saying in a statement: “We still feel betrayed that St. Paul’s School allowed and fostered a toxic culture that left our daughter and other students at risk to sexual violence. We trusted the school to protect her and it failed us.”

St. Paul’s rector Michael G. Hirschfeld commended “the remarkable moral courage and strength demonstrated by the young woman who has suffered through this nightmare,” and said the prep school is committed to teaching its students to act honorably.

Labrie was allowed to remain free on $15,000 bail while he awaits sentencing.

The aspiring minister and captain of the school soccer team testified that he and the girl made out, but he said he stopped short of intercourse because he suddenly decided “it wouldn’t have been a good choice for me.”

He acknowledged bragging to friends that he had intercourse with the girl, but he said that was a lie told to impress them. He also admitted deleting 119 Facebook messages, including one in which he boasted that he “pulled every trick in the book” to have sex with her.

In graphic and sometimes tearful testimony, the girl, now 16, said she willingly went with Labrie to the rooftop of an academic building after he invited her to take part in Senior Salute, a tradition she knew about. But she said she was prepared for kissing at most.

She said that Labrie soon become aggressive and that she told him, “No, no, no” as he moved his face toward her crotch. She said he eventually penetrated her, and she felt “frozen” — incapable of moving or reacting.

“I tried to block out the feeling as much as I could,” she said. “I didn’t want to believe this was happening to me.”

Under cross-examination, she said she helped Labrie remove her shirt and pants. And she acknowledged exchanging breezy email and Facebook messages with Labrie in the hours afterward, saying she kept the conversation light because she was trying to find out whether he had worn a condom.

After Labrie’s arrest, St. Paul’s announced it would expel anyone participating “in any game, ‘tradition,’ or practice of sexual solicitation or sexual conquest under any name.” The school, which first admitted girls in 1971 and has about 530 students, also brought in experts to discuss such topics as harassment and relationships.

Carney, Labrie’s lawyer, said the young man’s boasts about having sex with the girl amounted to the most damning evidence against him. He said Labrie was convicted because he wasn’t mature enough to tell his friends the truth.