CNN — A union representing school workers in Los Angeles plans to strike for three days next week with support from the district’s teachers union, it said, setting up what could be a shutdown of the nation’s second-largest school system.

Cafeteria staff, bus drivers, custodians, teachers’ assistants and other members of SEIU Local 99 – Education Workers United – voted to approve a strike next Tuesday through Thursday after nearly a year of negotiating with the Los Angeles Unified School District, it said.

United Teachers Los Angeles, which represents about 35,000 educators in the district, said it will honor the strike and not cross picket lines, the school workers union said in a news release.

If the union strikes, “it is virtually impossible to keep schools open,” district Superintendent Alberto Carvalho said during a news conference Wednesday morning. The district serves more than 600,000 students at more than 1,000 schools, its website says.

The district was last shut down by a six-day strike in 2019 as the teachers union demanded smaller class sizes, more school staffing and higher wages. Other districts across the country, including those in Seattle, Minneapolis and Columbus, Ohio, have also dealt with strikes in recent years as educators and school workers demand better working conditions.

In Los Angeles, the school workers union wants “equitable wage increases, more full-time work, respectful treatment, and increased staffing levels for improved student services,” it said.

Members have been negotiating with the district since April, the union said. In December, talks reached an impasse, and a state mediator was brought in, the school workers union said. Then last month, 96% of the union’s 30,000 district employees voted to authorize a strike, it said.

“As LAUSD parents and workers, SEIU Local 99 members know a strike will be a sacrifice but the school district has pushed workers to take this action,” Max Arias, the union’s executive director, said in a statement.

The school workers’ average salary is $25,000 a year, and most work part-time, which has led to staffing shortages, according to the union.

“Families have been sacrificing for far too long on poverty wages. Students have been sacrificing for too long in school environments that are not clean, safe or supportive for all,” Arias said. “Too many workers have been subjected to harassment simply for demanding change. Enough is Enough!”

Carvalho, however, said the union is “simply refusing to negotiate” despite a “historic offer on the table that was created in direct response to SEIU’s demands.”

“With both time and resources to be allocated in reaching an agreement, we are calling on them to come to the table for staff and students, right now,” the superintendent said in a statement shortly before the strike was announced. “We need to reach a resolution that honors the work of our dedicated employees, while respecting the rights our children have to a quality education, meals and access to enriching school activities.”

The district is preparing for school closures, Carvalho said. “We will be ready in less than ideal conditions.”