Teachers Matter - all of us know that (I hope), but research by Raj Chetty and John Friedman of Harvard University and Jonah Rockoff of Columbia University shows the long term impact of teachers on students. Having a good teacher benefits a child long after they have had that teacher. The one issue I have with the study, is that they define "value added" as the average test score gain for those students (with some adjustments for differences across classrooms).
It is interesting to look at though. (See the executive summary or full presentation video.)