This post from Alex Tabarrok came out a while ago, but I want to mark it:

The Times obtained seven years of math and English test scores from the Los Angeles Unified School District and used the information to estimate the effectiveness of L.A. teachers — something the district could do but has not.

... After a single year with teachers who ranked in the top 10% in effectiveness, students scored an average of 17 percentile points higher in English and 25 points higher in math than students whose teachers ranked in the bottom 10%. Students often backslid significantly in the classrooms of ineffective teachers, and thousands of students in the study had two or more ineffective teachers in a row.