In 2004, Chicago ratified a new collective bargaining agreement that made it a lot easier for principals to remove teachers, particularly non-tenured teachers, who enjoyed a number of protections. Over the next couple of years, more schools began dismissing teachers, finds University of Michigan researcher Brian Jacob. For example, the number of first year teachers who did not return for a second year, either because they were dismissed or elected not to return, jumped from 10 percent to 19 percent.

Still though, roughly a third of the schools' principals did not dismiss any teachers, and principals in the lowest performing schools were more likely to be in this group. This reinforces the theory that it's not contracts that prevent principals from firing teachers, but culture: that isn't the way it's done in schools.

Oddly, half of the teachers who were let go were rehired the following year by another school in Chicago, indicating that some principals either don't trust their colleagues opinions or are just too soft. In any case, some came to regret their decision, firing these rehires at a higher rate than any other group of teachers in the system.

And there's plenty of evidence that the culture is going to be tough to change. Most Chicago principals are still as generous as can be with their evaluation ratings. In 2007, only 15 teachers out of Chicago's 12,000 teacher workforce earned anything below a "satisfactory" rating, with 94 percent rated not just satisfactory, but excellent or superior.

The good news is that Chicago principals who did take on dismissals seem to be targeting the right teachers. Those teachers were absent more often, had lower value-added scores and lower evaluation ratings. They also were more likely to have failed their licensing tests st least once and gone to less selective colleges, two measures which have been found to correlate with teacher effectiveness.