When I was a high school student, some teachers explain me understandablely and I could understand what point I mistook.   Other teachers' explanation was too complicated to understand.  I would thought that the teachers did not know that they needed to change their way of teaching.     Students' evaluation of teachers might become a good chance for teachers to realize how their students understand their lecture and to improve their way of teaching.   I admit this point.  However, I disagree with it because of its disadvantage that is massive to its advantage. 
  

   Students' evaluation isn't done only based on teachers' skill of teaching, which might be voting of popular teacher.   There are many kinds of students and some of them do not pay attention to improvement of their own academic ability.  If only teachers entertain them or let them do what they want, the teachers are "good " teacher for them.   Of course many students enthusiastic about study will evaluate teachers depending on their teaching skills or their eagerness to education.  But teachers might learn to fare for bad marking on students' evaluation.  They might spoil their students instead of warning or giving them bitter advice.  This is bad for both teachers and students. 
 

    Without good lecture of teachers, students can improve their academic abilities for themselves or in other place.  My classmate in my university learn at a commercial high school and had to learn lots of other subjects than her essential subjects for entrance examination of my university.   She could not afford to learn at prep school and was forced to learn her necessary subjects only by herself.   But she was not defeated the diversity and passed entrance exam, without receiving good lecture.  She said it was miracle but I thought it was due to her great effort.  Like my classmate, if only students have strong incentive to study, their academic abilities would be inproved without good teacher.   As you know already, in prep school, professional lecture teach students subjects effectively.   Since they are familiar with entrance exam and can concentrate on teaching, teachers can take adequate lecture depending on their own necessity.
  

    In conclusion, I never the good result of evatuation of teachers.  But again, it can prevent teachers from teaching other things than subjects.  Teachers have to teach students common sense or importance of cooperation with others.  There is no prep school that teach students such kind of things, which is essential for them to live as an independent citizen in the society in the future.