About half of the teachers in Denver Public Schools ditched students for the picket lines Monday – the first strike in a quarter century – over a pay dispute that largely boils down to about $300 per teacher.

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Denver schools pays teachers a base salary, plus a suite of bonuses worth thousands more for educators in hard-to-staff positions, or those who take on more challenging roles in high-poverty schools. The Denver Classroom Teachers Association wants to roll back the “ProComp” bonus system to divert more teacher pay to the traditional seniority-based system, which offers automatic annual pay increases and raises for college credits with little consideration for district or student needs.

The ProComp plan, initially developed with buy-in from the union 20 years ago, also incorporates student learning into how the district distributes $33 million in tax money for teacher compensation, so neither the union nor the district want to scrap the program entirely, the Denver Post reports.

DPS and DCTA negotiators spent the last 15 months exchanging proposals for the new union contract until union negotiators walked out of talks over the weekend. The district’s most recent $20.8 million pay plan would cut down the incentives but retain $2,500 bonuses for teachers in high poverty and high priority schools, as well as a $750 annual bonus for teachers in 10 “distinguished schools,” with criteria for those schools designed by a district team.

The union’s latest proposal would cost $28.4 million, distributed mainly through the DCTA salary schedule based on seniority, college credits and professional development. The union plan includes a $1,750 annual bonus for teachers in Title I schools and a $2,500 bonus for hard to fill positions, the Post reports.

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According to NPR:

A starting teacher salary in Denver currently begins at $43,255 a year, USA Today reports. The district proposal raises that to $45,500, but teachers want $45,800.

At least 2,100 of Denver’s more than 5,000 teachers walked out on students Monday over the $300 difference, KDVR reports.

The strike will impact 71,000 students in 147 schools across the district, though all will remain open with the exception of preschool classes. District officials reportedly added 300 substitutes to a roster of 1,200 in preparation for the strike. About 1,400 administrators are also taking to the classroom, according to the Post.

Gov. Jared Polis told The Colorado Sun the strike will cost about $400,000 a day. He held a press conference to summarize the current disagreement that highlighted the $300 pay difference.

“We will be watching closely as the parties return to the negotiating table,” Polis said, tamping down calls for him to block the walkout.

When, exactly, union negotiators will return to the bargaining table is unclear. Union Treasurer Rachel Sandoval told Colorado Public Radio DCTA negotiators are taking time to “cool off” and rest.

“It’s been a very long time of negotiating and right now they just feel like there’s no give, actually regressing, and so they’ve just asked for some time to cool off,” she said.

“If you’ve been to our bargaining, you’ll notice that a lot of our bargaining team, they’re incredibly sick. One of bargainers, her child cried for most of the day so dad brought him and she held her son at the bargaining table yesterday,” said Sandoval, who got a head start with other DCTA protestors Sunday in front of the DPS building. “Our teachers, you know we want to say students first, but what about us as well? Our bargainers are exhausted, they’re sick, they need time with their family.”

In the meantime, DPS teachers are toting picket signs and chanting about money outside of their schools, following the example of teachers in Los Angeles who recently won a new unsustainable contract through a week long tantrum last month.

The Denver strike is also reportedly causing mayhem in schools, where students are posting videos of dance parties and mass walkouts as substitutes struggle to control the situation.

“There wasn’t much control,” student Matt Pence told the Post. “The substitutes were trying as hard as they could, but there were just too many people.”