Those who support students' criticism or evaluation of teacher tend to argue that such efforts will lead to an improvement in teaching and learning quality. Their reasoning is that under pressure teachers will have to work harder and produce results. On the one hand, there is no hard evidence in support of their reasoning. On the other hand, students' random or non-constructive criticism and evaluation of a teacher's work can be counterproductive, causing a loss of respect and discipline in the class.
I believe that if the major purpose is to enhance the quality of teaching or learning, many things should be done. Teacher's working conditions have to be improved first. Better compensation packages should be made available, and training opportunities offered. Culture is an important stuff which must be considered before deciding whether or not to urge students to assess a teacher's performance. Even in the west with long standing traditions of democracy, such an approach may not work: the face and respect of the teacher are all too important variables to ignore; whereas in Asian nations where democratic processes are just beginning to flourish, the time has not come to do it. The class can turn dysfunctional where the teacher is no longer respected.
In conclusion, while it is tempting to allow students' critical assessment of a teacher's work, such an idea should not be encouraged, and the cultural factor need to be addressed, as well