This one really burns me.
My sister, a junior in high school, was watching another of our sisters in her spring play, when a classmate started goading her about why she didn't believe in global warming. After giving an answer, several other students started berating her about it. Finally, when she tried to leave, one asked her, "Why are you a Republican?" Sick of the whole thing, my sister said--awesomely--"Because I have eyes to see!" They continued to accost her, and she said that she'd tried to explain it to them for years. The teacher who was in charge of the play walked in at that moment and said, "That's the problem with rational arguments; you can't stand up under them."
This is a fantastic statement considering the anti-rational means of spreading the religious faith and zealotry of liberalism employed by this guy: a few students boisterously gang up on another--and, by the way, much better--student, then a teacher walks in and backs up those other students without recourse to rational debate. This is the same teacher that has been spouting his opinions and (often wrong) facts in class as though they were true: global warming is real, corporations are evil, religion's got it all wrong, and Alger Hiss was innocent. (That last one is especially hilarious, considering that the great Whittaker Chambers used Soviet documents to prove that he was in fact guilty of spying for the Soviets.)
This sort of behavior--and, to be completely frank, historical ignorance--is unacceptable in a high school teacher.