When we select a school for our children we see how are the results of that school in the previous year and how is the feedback from the parents of the students who are already studying in the school. That means we are asking the opinion of the parents on the teachers of that school and nothing wrong with that.
But a student should not criticise the teacher and his subject knowledge. The person who tries to analyse another person should have the knowledge and ability. A student may not have that. But the parents of the students may have a piece of better knowledge and understanding about some of the subjects and they may be able to judge the teacher.
Knowledge is power.