CHICAGO — Thousands of teachers walked off the job Monday in Chicago’s first schools strike in 25 years, after union leaders announced that months-long negotiations had failed to resolve a contract dispute with school district officials by a midnight deadline.

The walkout in the nation’s third-largest school district posed a tricky challenge for the city and Mayor Rahm Emanuel, who said he would push to end the strike quickly as officials figure out how to keep nearly 400,000 children safe and occupied.

“This is not a strike I wanted,” Emanuel said Sunday night, not long after the union announced the action. “It was a strike of choice … it’s unnecessary, it’s avoidable and it’s wrong.”

Some 26,000 teachers and support staff were expected to join the picket. Among teachers protesting Monday morning outside Benjamin Banneker Elementary School on Chicago’s South Side, eighth-grade teacher Michael Williams said he wanted a quick contract resolution.

“We hoped that it wouldn’t happen. We all want to get back to teaching,” Williams said, adding that wages and classroom conditions need to be improved.

Officials said some 140 schools would be open between 8:30 a.m. and 12:30 p.m. so the children who rely on free meals provided by the school district can eat breakfast and lunch, school district officials said.