Denver teachers went on strike Monday after failing to reach a deal with administrators on pay.

The school district said schools will remain open during the strike and will be staffed by administrators and substitute teachers. However, the district has canceled classes for 5,000 preschool children because it doesn’t have the staff to take care of them.

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.

At a press conference Monday morning, union leaders expressed frustration at failed talks to reach a deal over the weekend. 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 main sticking points in the talks over a contract governing Denver’s incentive pay system, which started over a year ago, 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 things like 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.

The strike is the latest action in a wave of teacher activism since last spring, when teachers walked off the job in West Virginia.

Other recent teacher demonstrations, like the teacher walkout in Los Angeles last month, focused on more than pay, such as reducing class size and other issues more directly related to students.

However, 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 eat up 1 or 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 new Gov. Jared Polis, a Democrat, decided Wednesday 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 a walkout dragged 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 participate in a fact-finding process