Evaluate it based on the effectiveness and willingness of a teacher to teach and less on the grades earned by half-a$$ed students who don't care.
A good teach can get bad students. Plus it causes the teach to focus more on whether they can take a test instead of learning. There is also the entire thing of students possible blackmailing the teacher by saying they would purposely flunk the test unless "some demand".
Paying someone based on how well someone else does is kind of ridiculous, even if that person has somewhat of an impact. I mean if the teacher slacks off and doesn't give a sh*t, then fire them. But there are kids that don't and won't try, no matter what.
Unless it accounts for socioeconomic factors + other variables.
A great teacher ends up teaching the worst class, gets low pay. A horrible teacher teaches the elite's class, gets high pay in spite of performance. That isn't fair.
Think about it. Education is already dwindling before our eyes. What is Merit pay? It is pay based on a teacher's performance. Most teachers, being in one of the rougher jobs today, are ready to compete to earn. Why would a very good teacher, with good methods, who has been teaching a while be willing to help out a new, young, inexperienced teacher when he or she could become hard competition. If a teacher is benefiting from Merit pay, it is only logical that he or she will keep to themselves, avoiding to share their techniques, or methods that are why they are recognized as an honorable teacher.
Ever notice how, when a school goes from mostly white to mostly black, it becomes a "failing" school? Did the teachers get worse or did the students? If pay was based on results, bad teachers in suburban schools would make more than good teachers in ghetto schools.
Merit pay is trying to boil down a complicated equation to one simple thing: teachers. The reality is is that there are more difficult classes and less difficult classes, the demographic of the students-socioeconomic, race and ethnicity, religions, etc, that can affect the outcomes of standardized tests, what about testing anxieties and personal problems going on in the lives of the children (i.e. If a kid found out the morning of the test that grandma died, he/she probably isn't going to do well). There can be biases found on the tests, etc.. There's just too much going on that can affect the outcomes of testing that blaming the teacher for all of them is just wrong.