Teachers get first hand experience at how messed up the public education system is.
Even when I was in school they didn't teach anything, it was a simple memorization of the "facts" at that time. I've noticed that even with a couple years difference between my wife and mine education the number of differences in the history books as facts are changed to match the authors views of the event and the teachers are required to teach it even if they know it was wrong.
There were times when I took my own books to school and read them in class when the teachers wouldn't teach, the ones that would take us down to the gym so they could spend time with their sports team getting them ready for the big game while their class had a study hall.