yeah that is a problem and a good point. Also if you did it by testing, you would have teachers feeding students the answers or teaching for the test instead of teaching them what they need to know like is already happening with the current government testing. I don't think it should be up to the students though, because then it would just be a popularity vote. One of the best teachers I ever had, most students hated because she was strict. They never understood why I liked her. It was because she was teaching me math that I was told was supposed to be hard to learn but it was easy in her class because she was a great teacher.
In general, though, I don't think now is the time to worry about teachers and other government employees making enough. Everyone is taking big pay cuts and they are relatively safe. Another perk with the job -- job security.