I think it's a good idea for schools to ask students to evaluate their teachers. This informs teachers of how students react both to their teaching methods and to them as individuals. Teachers can weigh the criticisms and change what they think needs to be changed. Praise from students is also valuable. It can reinforce teachers' opinions about how they're teaching and give them confidence.
It's also helpful for school administrators to hear what students think about their teachers. It keeps administrators in touch with what's going on in the school. It also gives them some idea of which teachers effectively reach the students. However, good administrators know that student evaluations need to be reviewed with a keen eye for prejudice and adolescent attitudes. After all, and evaluation is a good way for students to get back at teachers who expect more of them than they want to give.
Evaluating teachers is also a good exercise for the students. They have to organize their thoughts about what they think of their teachers. In that way, they pinpoint for themselves what they expect of those teachers. They begin to understand what they value in a teacher and what is phony or useless. Evaluating their teachers is also a way for students to think about how they would evaluate themselves. Are they working as hard as they should in class? Is that why their teachers are sometimes critical of their work? Isn't that what a teacher should do—push us to try harder? Thinking about these things can help students do better in class. At the very least, they will understand better what is expected of them.
Student evaluations of teachers also make students feel as if they have a voice in what happens in their schools. It makes them feel as if they're part of the education process when their opinion is valued by the administration. Finally, it teaches them responsibility.