Time and time again, people suggest that violent video games cause people to become violent. This is always refutted by scientific evidence. "Correlation indicates the possibility of a cause-effect relationship, but it does not prove causation." <--- Quote by David G. Myers (He has a Ph.D from the University of Iowa. He also wrote our current College Psychology book.)
Yes, students should be able to grade their teachers. If students were allowed to grade their teachers, it would creatively reflect how well the teacher is explaining and presenting the class material, and how well the students are understanding the material.
However, if the students aren't mature about the grading system, it could become a popularity contest, or if the teacher teaches a "sluff" class, some students may give that teacher the highest grade.