Everybody now says we must evaluate teachers based on how much value they add to their students' performance, but nobody has actually done this. It can take a long time to work out a good statistical system -- e.g., it took Bill James 25 years to work out his Win Shares statistic for evaluating baseball players. Michelle Kerr, a teacher who thinks statistically, takes a first crack at some of the necessary conditions in this Washington Post op-ed.
Another tricky issue is what to do about teachers' aides, special ed aides, handicapped students' companions, and other adults who might be in the classroom with the teacher. The more adults the better (hopefully), but not all adults are of equal value to the teacher. Under a value-added system of pay, teachers will finagle even more than now to get the most useful aides in their classrooms. Those who win the political battles for the most and best aides will get paid more unless the aides' inputs can be taken into account.