Broward’s school district has made great strides in complying with state class-size requirements, but one of the techniques it used — forcing high schools to adopt a uniform, seven-period class schedule — violated its teachers union contract, an arbitrator has found.

Because Broward “completely ignored” contract provisions that allow teachers to keep their school’s current schedule if they preferred, the district must return to the schedules it used during the 2011-12 school year, arbitrator David Weitzner ruled.

During that year, only a handful of Broward’s 29 high schools used the seven-period schedule. A dozen Broward high schools that year used the popular “block” scheduling format — students take four 90-minute class periods for one semester, and then take four different classes for the second half of the year.

Some parents favor block scheduling because it allows students to focus on just a few subjects at a time. But Superintendent Robert Runcie argued last year that switching to seven periods would help reduce school class sizes. The expected benefits were twofold: The new, more-efficient schedules would save money (allowing the district to hire more teachers) and would also create greater scheduling flexibility that could help lower class sizes.

The district made progress on class size, with more than 87 percent of classes complying with state rules in 2012-13, compared to just more than half complying the year before. Not all of those gains, however, can be attributed to the new high school schedules, as the district employed a wide variety of strategies to shrink its classes.

During the arbitration battle with the union, school district leaders argued that state law allowed the district to alter high school schedules without teacher approval. Weitzner, in his binding decision against the district, said Broward must honor the terms of its teacher contracts just as it would “a contract with a general contractor to renovate or build a new school building.”

In a brief written statement, the school district said it was still “conducting a thorough analysis” of the ruling and its impact. The district stopped short of confirming that block schedules will return.

“At this time, there are no changes to current schedules,” the statement said.

Steven Feldman, a Broward Teachers Union field staff representative, said the district’s teacher contract requires two-thirds of a school’s faculty to support any class schedule changes. At many high schools, that level of support for a seven-period system just wasn’t there last year, yet Broward went ahead and made the changes anyway.

“The teachers feel that they should have the say in which schedules are best for the students,” Feldman said.

In many cases, the forced change to seven periods increased teachers’ workloads.

“It’s been very, very, very tough on our teachers, we acknowledge that,” said Cynthia Park, Broward’s director of college and career readiness, in a presentation to School Board members earlier this year.