Today's New York Times Magazine asks that question, weighing in on recent developments in real-life teacher accountability: 

A few months ago, a team of reporters at The Los Angeles Times and an education economist set out to create precisely such a consumer guide to education in Los Angeles. The reporters requested and received seven years of students’ English and math elementary-school test scores from the school district. The economist then used a statistical technique called value-added analysis to see how much progress students had made, from one year to the next, under different third- through fifth-grade teachers. The variation was striking. Under some of the roughly 6,000 teachers, students made great strides year after year. Under others, often at the same school, students did not.

See the full magazine article here.