PORTLAND, Ore. (AP) — Teachers in Portland, Oregon, walked off the job on Wednesday for the first day of a strike that will shutter schools for some 45,000 students in Oregon’s largest city.

Concerns over large class sizes, salaries that haven’t kept up with inflation and a lack of resources prompted the strike, one of the latest signs of a growing organized labor movement in the U.S. that’s seen thousands of workers in various sectors take to the picket lines this year.

The Portland Association of Teachers, which represents more than 4,000 educators, said it was the first-ever teacher’s strike in the school district. The union has been bargaining with the district for months for a new contract after its previous one expired in June.

Portland Public Schools did not respond to requests for comment on Tuesday night or Wednesday morning.

Schools are closed and there is no classroom or online instruction during the strike.

Mike Bauer, a union representative and special education teacher at Cleveland High School, said teachers were stressed about the strike but felt it was the right way to advocate for their students. He said that smaller class sizes would both lighten educators’ workload and help them give students more individualized attention if they’re struggling.

“It’s about the kids,” said Bauer, who’s been teaching in Portland for nearly 20 years. “It’s about the sustainability of the job and the longevity of our jobs.”

Questions of pay — particularly for teachers just starting their career — have also been raised as the cost of living has increased in Portland, he said. The annual base salary in the district starts at roughly $50,000.

“I’ve seen many people quit within their first five years,” he said. “At the end of the day, we need teachers.”

Nearly two weeks ago, the union announced that 99% of teachers voted in favor of the labor action, with 93% of its members participating in the ballot.

After the union voted to authorize the strike, the district said it wanted to reach a fair settlement. “We ask our educators to stay at the table with us, not close schools,” it said in an emailed statement on Oct. 20.

Oregon Gov. Tina Kotek had urged the union and the school district to come to an agreement and avoid a walkout.

Public education has been gripped by a series of high-profile strikes this year.

In the Los Angeles Unified School District, the nation’s second-largest, workers including teachers’ aides, cafeteria workers and custodians walked out for three days in March to demand better wages and increased staffing, shutting down education for half a million students.

In Oakland, California, the union representing teachers, counselors, librarians and other workers went on strike for more than a week in May. In addition to typical demands such as higher salaries, it also pushed for “common good” changes, such as reparations for Black students and resources for students who are homeless.

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Claire Rush is a corps member for the Associated Press/Report for America Statehouse News Initiative. Report for America is a nonprofit national service program that places journalists in local newsrooms to report on undercovered issues.