Teachers are at the center of the great debate over how to fix American education. We're told the bad ones need to be fired; the good ones, rewarded. But what about the rest? Most teachers are in the middle — not terrible, but they could be better. If every student is going to have a good teacher, then the question of how to help teachers in the middle must be part of the debate.

One reason "teacher improvement" doesn't get more attention is because researchers don't know that much about how teachers get better. Typical professional development programs, in which teachers go to a workshop for a day or two, aren't effective. Even programs that provide longer-term training