I think it is more psychological and a multitude of reasons. 1) Everyone went to school, therefore everyone is an expert on teaching. 2) Teachers are one of the few public employees that are accessible to the average citizen. An outlet for displeasure against the government. 3) Its largely a female profession. 4) If your child isn't doing fantastic, it's obviously the teachers' fault. 5) Teachers by and large vote one way, inviting the ire of people who vote the other way.