The Chicago Teachers Union says its members will go on strike Monday for the first time in 25 years.
The union says contract talks with the district failed late Sunday night over issues including benefits and job security.
"We will be on the (picket) line," Chicago Teachers Union President Karen Lewis said Sunday night, calling it a difficult decision and one the union hoped could be avoided.
More than 26,000 teachers and support staff are expected to hit the picket lines Monday morning, while the school district and parents make plans for keeping students safe and occupied during the day.
School board President David Vitale announced a short time earlier that talks had broken off despite the school board offering what he called a fair and responsible contract that would cover four years and cost the city $400 million.
Lewis said she believed talks would resume Monday but a time had not been set for the sides to meet. She added that progress had been made but not enough to avert a strike.
The strike was announced after months of tense, at-times heated talks between the city and the union representing 25,500 public school teachers.
The district, the third largest in the United States, had been offering a raise of 2 percent a year for four years. The union called that offer unacceptable -- particularly after Mayor Rahm Emanuel last year canceled a previously negotiated 4 percent pay raise, citing budget problems.
The union countered by asking for a 30 percent pay raise over two years, followed by a request for a 25 percent increase over two years. Just weeks ago, Lewis told delegates the union had adjusted its demand and was asking for a 19 percent pay raise in the contract's first year.
The union also has raised concerns about raises based on teacher experience and education. It said the district agreed to retain contract language allowing raises based on experience, called step increases, but would not actually pay the money now.
Teachers also have been concerned about new teacher evaluations, health benefits and regaining lost jobs. An additional issue was how a longer school day for students is being implemented.
The strike is the latest flashpoint /The threatened strike was the latest chapter in a very public and often contentious battle between the mayor and the union.
When he took office last year, the former White House chief of staff inherited a school district facing a $700 million budget shortfall. Not long after, his administration rescinded 4 percent raises for teachers. He then asked the union to re-open its contract and accept 2 percent pay raises in exchange for lengthening the school day for students by 90 minutes. The union refused.
The longer school day was one of the mayor's campaign promises for the city's schools, and he pushed to have it implemented a year ahead of schedule. He attempted to go around the union by asking teachers at individual schools to waive the contract and add 90 minutes to the day. He halted the effort after being challenged by the union before the Illinois Educational Labor Relations Board.
The district and union agreed in July on how to implement the longer school day, striking a deal to hire back 477 teachers who had been laid off rather than pay regular teachers more to work longer hours. That raised hopes the contract dispute would be settled soon, but bargaining continued on the other issues.