I have a bunch of these from elementary school and a few from high school.
When I was in elementary school, I used to have a teacher who got really mad at me for spelling the word "colour" with a u (when we had books that had that spelling).
My first grade teacher also hated when I used the word "vague" in my writing, despite me knowing what it meant. I was in first grade and didn't have the balls to use it again until third or fourth when I saw it in a book and thought it was "un-banned". Then again, said teacher wanted me to be held back because she thought I was severely disabled and not "emotionally ready" (when in reality the school staff didn't take bullying that seriously) for elementary school.
I also got yelled at for looking at a poster. The teacher thought I was looking at the "bad child" of the class. The way it was set up was that there were a series of posters in the closets where we'd all put our backpacks, and so if I wanted to I could turn around and read the posters. She interpreted it completely differently, and began to yell at me for "encouraging" the kid to continue misbehaving, and threatened to send me to the principal's office. Being the good noodle I was, I began to cry in the class because I didn't want to get in trouble.
And now jumping to high school, the sophomore class president was very conservative and actually somewhat pro-Hitler. He said Hitler had the right ideas, and that if he (the kid) could he'd gas anyone from the Middle East and put the gays in insitutions. The times I called him out for being racist he'd always threaten to shoot me - very loudly - which the school... didn't think was a problem, somehow? In public school I'm pretty sure the kid would've been expelled. Catholic school was a strange place.