The union representing Chicago teachers is to continue its week-long strike, extending its bitter battle with the mayor over evaluations and job security that are central to the debate over the future of public education across the United States.
Mayor Rahm Emanuel said he would seek a court order to end the strike, which he said was illegal under state law.
Union delegates declined to vote formally on a proposed contract settlement worked out over the weekend with officials from the nation's third largest school district and schools will remain closed.
Union president Karen Lewis said teachers wanted the opportunity to continue to discuss the offer on the table. "Our members are not happy," Ms Lewis said. "They want to know if there is anything more they can get." She added: "They feel rushed."
She said the union's delegates would meet again on Tuesday and the soonest classes are likely to resume was Wednesday. "We felt more comfortable being able to take back what's on the table and let our constituents look at it and digest it. We can have a much better decision come Tuesday," said delegate Dean Refakes, a physical education teacher at Gompers Elementary School.
But Mr Emanuel said the strike issues were "deemed by state law to be non-strikeable". "This was a strike of choice and is now a delay of choice that is wrong for our children," he said in a statement.
The walkout, the first in Chicago in 25 years and the first for a major American city in at least six years, halted classes for 350,000 children who just returned from their summer holiday. It forced tens of thousands of parents to find childcare alternatives, including many whose neighbourhoods have been wracked by gang violence in recent months.
The strike drew national attention because it posed a high-profile test for teachers unions, which have seen their political influence threatened by a growing reform movement. Unions have pushed back against efforts to expand charter schools, bring in private companies to help with failing schools and link teacher evaluations to student test scores.
The strike carried political implications, too, raising the risk of a protracted union battle in President Barack Obama's home town at the height of the autumn election campaign, with a prominent Democratic mayor and Mr Emanuel, Mr Obama's former chief of staff, squarely in the middle.
Mr Emanuel condemned the teachers' decision to leave classrooms, calling the walkout unnecessary and a "strike of choice".