CHICAGO –  As teachers in the nation's third-largest school district strike this week, one of their biggest sticking points is how much student test scores should be tied to their own job evaluations.

It's a complex and charged issue playing out in school districts nationwide. Boston and Los Angeles just agreed to new assessments taking student performance into account, and Cleveland is already implementing a new approach.

But with high stakes the tone of how districts and unions have reached agreements has varied wildly. In Los Angeles courts have intervened. In Boston, it took two years to reach a deal. In Cleveland, the new evaluations are taking four years to implement.

In Chicago, roughly 25,000 teachers are expected to remain out of the classroom for a fourth day Thursday.