Teachers are not always right... and students are not always stupid and misbehaving.
I realize there is a mystical "teachers can do no wrong" feeling when it comes to education, but there are many many many many cases of them stepping out of line (I have had a few too, who lowered my grade for their mistakes, such as actually losing my work after I had turned it in).
Teachers are human too. There can be good and bad.
This is the same attitude that allowed priests to molest kids for so long unquestioned... bleh. Just because they are in a position of authority or are older, doesn't mean they are incapable of acting in wrong ways.