Chicago's teachers went out on strike this morning for the first time in 25 years after contract talks with school authorities collapsed.
The walkout followed months of talks between Mayor Rahm Emanuel and union leaders on issues including compensation, health benefits and teacher evaluations.
More than 26,000 teachers and support staff hit the picket lines today, while the school district tried to keep nearly 400,000 pupils occupied.
"This is a difficult decision and one we hoped we could have avoided," Chicago Teachers Union president Karen Lewis said.
"But we must do things differently in this city if we are to provide students with the education they deserve."
Ms Lewis said progress had been made but not enough to avert a strike.
Union officials said the outstanding issues included standardised student testing that would "cheapen" the school system and a teacher evaluation system that would cost 6,000 teachers their jobs within two years.
The district had been offering a raise of 2 per cent a year for four years.
The union called that offer unacceptable and countered with a demand for a 19 per cent raise in the contract's first year.
When he took office as mayor last year, virtually the first act of the former White House chief of staff was to scrap 4 per cent raises for teachers.
Mayor Emanuel then asked the union to reopen its contract and accept 2 per cent pay raises in exchange for lengthening the school day for students by 90 minutes. The union refused.