In many years of schooling, I have witnessed on a daily basis certain teachers coming to school and not teaching. For example, a few teachers I knew didn't teach as if they were teachers, but they gave out work as if they were supervisors. This caused me to self teach at times. Also, I had to ask my peers for help. On the other hand, how do students know what its like to teach, or even how to do it?
That's a good reason why students should not be allowed to grade teachers.