CHICAGO (Reuters) - Chicago teachers went on strike on Thursday and set up picket lines outside many of the city’s 500 public schools, as union leaders and officials of the third-largest U.S. school district pushed ahead with negotiations over a new contract.

The work stoppage forced officials to cancel classes for more than 300,000 students, but school buildings stayed open for children who need a place to go during the strike.

Thousands of people including teachers and some parents who support their cause walked the picket lines, many of them wearing red shirts, carrying signs demanding “a fair contract” and chanting “Whose schools? Our schools.” Thousands of teachers and supporters also gathered downtown for an afternoon rally and march.

“I’m hopeful. I’m here to stand up for teachers and the future teachers,” said Pamela Wasson, a bilingual special education teacher in her 34th year of teaching as she picketed in front of her North Side elementary school.

The strike is the latest in a recent wave of work stoppages in school districts across the United States in which demands for school resources have superseded calls for higher salaries and benefits. In Chicago and elsewhere, teachers have emphasized the need to help underfunded schools, framing their demands as a call for social justice.

In addition to wage increases, the Chicago Teachers Union (CTU) wants more money to ease overcrowded classrooms and hire more support staff such as nurses and social workers, two perennial issues plaguing the district.

On Thursday afternoon, union President Jesse Sharkey said that union and district made progress at the bargaining table.

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“We have in our possession now something in writing about class size,” he said during a news briefing. “It’s a shame that it took a work stoppage to actually get that document.”

He did not specify the details of the class size proposal.

The district’s proposal would grant a 16% raise over five years, set enforceable targets for reducing class sizes and add more support staff across the district, according to Mayor Lori Lightfoot, who was elected in April.

“We certainly believe that we can get a deal done,” she said on Thursday.

Lightfoot has previously said that the union’s full list of demands would cost the district an additional $2.5 billion annually.

Wall Street credit rating agency Moody’s Investors Service, which rates the district’s debt in the junk level, said on Thursday that the outcome of labor negotiations will have “substantial ramifications on whether CPS’ financial recovery continues, given its limited financial flexibility and narrow reserves.”

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Teachers from Newton Bateman Elementary School on Chicago’s north side blanketed the blue-collar Irving Park neighborhood, waving signs and beating drums.

“This is not really a dollars-and-cents strike,” said James Phelps, a 59-year-old American history teacher. “The mayor literally campaigned on equal distribution of resources. Why hasn’t she agreed to that?”

PARENTS SCRAMBLING

A veteran of three strikes and a lockout over his 29-year teaching career, Phelps said swelling class sizes and a shortage of nurses and social workers at schools mainly on Chicago’s south and west sides justified the labor action.

Schools administrators were providing activities and meals at the schools while community organizations across the city promised to keep their doors open for children and parents.

“I’m just scrambling, looking for options,” said Tia Cochran, who was dropping off her son, a first-grader, at Newton Bateman Elementary.

The strike comes seven years after 29,000 Chicago teachers walked out for seven days over teacher evaluations and hiring practices. In 2016, teachers staged a one-day walkout to protest the lack of a contract and failures to stabilize the school system’s finances.