Good teachers improve performance of peers
The talents of great teachers appear to have a positive effect on their peers, according to North Carolina's report. Researchers looked at 11 years of state school data and found results suggesting that high-performing teachers' abilities not only resulted in better student performance in their classroom but in others at the same grade level. "If it's true that teachers are learning from their peers, and the effects are not small, then we want to make sure that any incentive system we put in place is going to be fostering that and not preventing it," said a co-author of the study.
Jackson, C., & Bruegmann, E. (2009). Teaching students and teaching each other: The importance of peer learning. Cambridge: National Bureau of Educational Research. http://www.nber.org/papers/w15202.pdf