Online websites such as ratemyprofessors.com are made for just such a purpose. Schools should have anonymous evaluations of teachers so principals and department heads can have a better grasp of what is going on in the classroom since supervisors can't be everywhere at once. Plus, it's only fair that students be able to turn the tables on their teachers and grade them based upon how they feel about a particular educator.
I had a teacher once who was very nice and well liked, but she was one of my worst teachers. She Never explained things very well and when you asked a question about an assignment she would not answer. When asked a my friend and I needed help with one of the worksheets she handed out (most work was done in groups). We asked her a question and she said "well what do you think?...Now do you think that's right?". We obviously didn't know if we were right, hence the question. We also discussed all our work with the class. With another teacher this could have been effective, but with her it usually went off topic or just didn't help us understand. When a student didn't do their work he didn't have to go in the hall, finish it before joining, or even just sit there and listen. He was allowed to be a part of the discussion and talk about the work, even though he said he had no idea what we were talking about. The same student was also frequently loud and disruptive, yet there were never any consequences. My friend almost failed the class because she lost all his work (no joke, she is the one who told him she lost the work) and I did fail the class because my work was lost. I really wish I had a way to grade her performance. As a person she was friendly and had a good personality, but as a teacher she was just terrible. Maybe if enough reviews were turned in something could have been done about it.
How many times have you heard someone say that he did poorly in a class because the teacher was hard to understand? Or the teacher was mean? Or the teacher was simply incompetent? Just because it's a student saying so, doesn't mean it's not true. Of all the people who should be reviewing teachers and how they teach, it's students. They know better than anyone.
Most students would not judge teachers on performance, rather they would judge them on how nice they ate and how much easy work they give, homework passes, etc.
There ARE teachers who are hard to understand. Unfortunately, I am not sure there is much to do about that. If a school wide survey was conducted about specific teachers, it would be best to compare the top students reviews with what the majority says. If they are one and the same, then steps would need to be taken to improve that teacher, or even fire her.