I can't see how quantifying merit would work unless we could also quantify the performance of parents, the child, and the community in its interaction with what the teacher does. Overall, we should be putting less emphasis on the teachers and more on the children and parents to get their butts in gear and stop expecting teachers to take up their slack.
I believe that until society values education again, its not really going to matter what we do. We should be celebrating scientists and engineers. Not athletes, talk show hosts, and music stars.