Teachers in Chicago, the third largest city in the U.S., went on strike Monday over a months-long dispute about how their job performance should be evaluated.

About 29,000 teachers and support staff walked off the job in the dispute, with some joining picket lines outside their schools. The strike shut 350,000 students out of their classrooms, forcing many working parents to stay home to care for their children or to quickly find alternative care.

Chicago Mayor Rahm Emanuel, a one-time key aide to President Barack Obama, blamed the teachers' union for the strike.

“I am disappointed that we have come to this point, given that even all the other parties acknowledge how close we are , because this is a strike of choice. And because of how close we are, it is a strike that is unnecessary.”

The city has offered the teachers a 16 percent pay raise over four years. But Emanuel said the city government and the teachers remain divided over increased authority for school principals and his proposal to base part of teachers' job performance evaluations on whether standardized test scores for their students are improving.

U.S. education officials have been advocating use of such tests for judging teachers, but teachers' unions have often opposed the idea. Union leaders say that many students in inner-city schools come from impoverished families, which makes it difficult for them to show quick improvement on the tests.