DENVER >> Denver teachers went on strike today after failing to reach a deal with administrators on pay in the latest example of educator discontent, following a wave of walkouts over the last year.

Denver’s teachers started picketing before the start of the school day and students crossed through the picket lines on their way to class in some locations. Students in at least one school walked out of class and demonstrated in support of their teachers.

The city’s schools will remain open during the strike and will be staffed by administrators and substitute teachers, the school district said. But canceled classes for 5,000 preschool children were canceled because the district does not have the staff to take care of them.

Union leaders told reporters they were frustrated with failed talks over the weekend aimed at reaching a deal. Union president Henry Roman said teachers were committed to reaching a deal but said that both sides needed a cooling off period. Another negotiation session is expected Tuesday.

“They need us. They need our labor, they need our minds, they need our talents to really make it happen,” lead union negotiator Rob Gould said.

The strike for Denver’s more than 4,000 teachers is their first in 25 years. It comes after teachers walked off the job in Arizona and West Virginia last year and Los Angeles teachers went on strike last month.

The Los Angeles teachers ended up getting the same 6 percent raise offered early on by the nation’s second-largest school district. However, they also sought and won promises for smaller class sizes and adding more nurses and counselors.

In Denver, the main sticking points in the talks over a contract governing an incentive pay system are lowering bonuses to put more money in teachers’ base pay and how to allow teachers to advance in pay based on education and training, the norm in most school districts.

The union pushed for lower bonuses for high-poverty and high-priority schools to free up more money for overall teacher pay and criticized the district for spending too much money on administration. However, the district sees those particular bonuses as key to boosting the academic performance of poor and minority students.

Some teachers argue that spending money on smaller class sizes and adding support staff, like counselors, is the best way to help disadvantaged students learn and make them good schools for teachers to work in.

Denver teachers say that the non-traditional pay system in the district leads to high turnover, which they say hurts students. They also hope that a win on pay will help them when it comes time to negotiate other issues when their overall contract expires in two years.

The state says a walkout will cost about $400,000 a day and would consume 1 to 2 percent of the district’s annual operating budget in about a week. In encouraging both sides to come to an agreement, Gov. Jared Polis has pointed out that this money will no longer be available to help pay teachers once it is spent on the strike.

While teachers in some states are barred from striking, teachers in Colorado have a qualified right to walk off the job. As required by state law, teachers gave notice last month that they planned to strike. But the walkout was put on hold because the school district asked the state to intervene.

The strike was on again after the administration of Polis, a Democrat, decided last week not get involved, believing the positions of both sides were not that far apart.

However, Polis said the state could decide to intervene — and suspend the strike for up to 180 days — if the walkout drags on.

The state does not have the power to impose any deal on either side. But it can try to help the union and school district reach a deal and can require them to participate in a fact-finding process