CHICAGO – Educators in Los Angeles just signed a new deal with the city's school district. So, too, did teachers in Boston. Both require performance evaluations based in part on how well students succeed, a system that's making its debut in Cleveland.

So what's the problem in Chicago, where 25,000 teachers in the nation's third-largest district have responded to an impatient mayor's demand that teacher evaluations be tied to student performance by walking off the job for the first time in 25 years?

To start, contract agreements in other cities have hardly come quickly or with ease. They were often signed grudgingly, at the direction of a court or following negotiations that took years. And mayors and school officials have also won over reluctant teachers by promising to first launch pilot projects aimed at proving a concept many believe is inherently unfair.