I think you make an important point Samir. Evaluations should necessarily follow some comments. It's rude to say that a teacher is not performing well in a certain facet without explaining why the student thinks so.
As for a teacher evaluation system, every campus should have a documented process of evaluating students and teachers. It cannot e left to a arbitrary feedback process. Think of developing a Teacher Evaluation System DFD (Data Flow Diagram). It will be useful to your campus management.