And now we're back to the fact that a teacher is only somewhat responsible for results. There are kids who are basically unteachable. They don't give a shit about you or your classroom rules. Put enough of them together and an extremely hardworking and talented teacher will get nowhere, and all the other kids will suffer as well.
This is why merit pay should be based on numerous factors, results being one of them, and non-tangible assets another. A teacher in the ghetto could work twice as hard, be twice as talented, as a teacher in suburbia, and never come close to the results.