Undecided questions
Should students evaluate their teachers?

Yes! It is a good idea because a school needs to know if the students can work with a teacher and learning can take place. Sure, some students might say bad things about a good teacher, but in the end the majority of students have similar ideas about teachers, and an evaluation gives the administration another piece of information about that teacher. . If most of the students say that the teacher didn't explain things clearly or went too quickly through the material, it's probably true.
It's good for the teacher, too, to see what students think is helpful and what is not. The teacher can change things to make the class better for future students.

yup

Here they do so already. The only problem is that if a student or a group of them decide they don't like a teacher or uni lecturer, they can combine to give them a poor rating.

some teachers yes ,, not all teachers

Yes sure, but being rational and fair is prerequisite to such an evaluation.By giving reasons why they dislike or like a certain teacher they are explaining their point of view.