My very first teacher who insisted on spelling my name wrong.
I have one of those names that's spelled differently for males and females. When I started school, we were presented with a card with our names on in bigggish writing and had to trace them. (I could already write my name and address, so found this pretty bloody pathetic, frankly).
Teacher wouldn't listen when I told her that my name wasn't spelt that way, and insisted I trace the wrong version when I could write the correct version without needing to trace it.
When my DM picked me up from my first day at school, I was livid and announced that I wasn't going to school any more because my teacher was stupid and wouldn't take any notice when I told her she was wrong. I remember feeling quite insulted that she wouldn't let a stroppy 5-year old correct her.
I'm also still pissed off that I didn't get put up a year. They were already to do it, because I was so far ahead of the others in reading and writing. Then towards the end of the summer term, the junior school I was about to switch to (infants and juniors were different schools where I lived) spotted that I had an August birthday, so would be almost 2 years younger than some of my classmates, decided that it was too big a gap and blocked it.
I wasted a whole year in the infants mostly reading and stuff on my own.
Still pissed off about it and it was nearly 60 years ago!