More than a dozen years ago, voters in Denver backed a measure to increase taxes to fund a novel experiment in teacher pay, offering educators bonuses based on performance and for going to work in high-needs schools and working in difficult-to-fill positions. At the time, the school district was a pioneer in performance pay, the first large urban school system to adopt that model.

Now, thousands of Denver teachers are striking to roll back the system, which they say has become unwieldy and unpredictable. Schools remained open Monday while teachers took to the picket lines, absences that are expected to affect the 70,000 students who attend traditional public schools. Students at one high school also walked out in support of the teachers. The school system canceled all early childhood education classes.

Denver took the lead on performance pay for teachers and was followed by several other large urban school systems, including in Dallas, Orlando and Washington. It's an issue that proved to be a lightning rod for teacher unions, who fiercely oppose performance pay systems. They maintained that it was based on measures that were unreliable and beyond the control of teachers, such as test scores and metrics that attempt to predict how effective teachers are at boosting test scores.

D.C. Public Schools has one of the most robust performance-pay systems. Adopted in 2009, the system tied teacher evaluations to test scores and established a pay structure that gives teachers who are rated as highly effective up to $25,000 in bonuses. But the system remains controversial, with critics suggesting that the fixation on data has driven scandals that have rocked the district.

The Denver Classroom Teachers Association, which represents about 5,700 teachers and other school professionals, has been pushing the Colorado school system to invest more in base pay and to curtail the bonus structure. The union has also sought to simplify it.

“We’re looking for a fair and reliable pay system that actually retains teachers in Denver,” said Rob Gould, a special-education teacher and lead negotiator for the teachers association. “We’ve had a 20 percent turnover in Denver year after year. The district — they have been doubling down on these bonuses that are unreliable.”

Denver Public Schools Superintendent Susana Cordova lamented that union officials walked away from the negotiating table Saturday. The strike comes at the end of 15 months of negotiations.

At a news conference Monday, Cordova said it would be foolish for the school system to scrap the bonus system — called ProComp — because it would mean forfeiting about $33 million that voters elected to set aside for the program in 2005. She said the district is working to simplify ProComp so teachers have a better handle on how much money they will make year after year.

"We are working to make it as transparent as possible," Cordova said.

The Denver teachers strike follows a year of educator activism that swept the nation, drawing teachers out of the classroom from West Virginia to Los Angeles, where teachers were on strike for a week to rally for more classroom resources. It appears likely it won’t be the last walkout, with teachers in Oakland, California, moving toward a strike.