Even constructive criticism about a teacher is usually not well received at schools. Schools tend to take a “us” versus “them” mentality as in the “school staff” versus “parents”. There is also the concern voiced by students that if the teacher finds out that a parent has been in to complain; the teacher will somehow take it out on the student in the form of low grades.
On the other hand if a parent does not complain it leads to anxiety, frustration, failure to learn and a lot of second guessing by the student and parent. It seems to be choice between a rock and a hard place!