I have a few options; my parents would definitely pick the (now deceased) teacher I had from age 6 to 8, when we moved away. In the first week, she divided all the kids into little angels and scum. Once you were in one group, it was a Herculean task to move out of the angel category, and impossible to cease being scum.
Aside from general needling and insulting, she never bothered me that much, (though I was definite scum), but she did some horrible stuff to other kids.
There was a girl in my brother’s class who basically all the parents watched out for; her parents were teens when she was born, and her mother just buggered off and literally left her on her 19-year old Dad’s doorstep. He certainly cared a lot about her, but he had no freaking idea how to care for a child. He used to wash her clothes by simply encouraging her to wear them in the bath. That sort of thing.
This teacher, for some reason, thought this amiable, always on the edge of foster care little girl was the worst child that ever drew breath. Anything that went wrong was blamed on her, and she’d get screamed at until she couldn’t stand up for crying.
One one occasion, she got the blame for breaking a photocopier, then the kid who actually had broken it, her next door neighbour, confessed to her mother that it was her fault. Both kids got dragged back to the school by the mother, so the one could confess, and the other get an apology. Apparently the bitch listened to the guilty kid, replied (the same person who screamed herself hoarse at the previous ‘guilty’ child) “Hm, well I’m surprised at you”, then walked off, completely ignoring the still crying kid she’d blamed.
She was actually the head as well, so it took a major effort from the PTA to get her fired, a few years after I left.
My second awful teacher, my physics teacher when I was 13-16, had an irrational hatred of me. Apparently each class had a scapegoat, and I missed the first class of term, due to being on holiday, (with full permission,) and that meant I was It. I had 4 detentions in the 7 years I was there, and three were from her- including a double one for not having my notes in the right order. I wasn’t allowed to sit with my friends, and she would take any opportunity to make me miserable.
I remember us having a mini test, and her coming over to hand out the results, and yelling at me for over 10 minutes- I’d missed the class we were being tested on due to being off ill, and had got something like 7 out of 20. Pretty poor… except when she handed back the paper to the girl next to me, she’d got 4/20. Her reaction to the other kid? A little laugh, and a comment of " Oh well, we all have bad days, dear". When she’d gone, we realised that out of the 4 of us sitting at the table, I had the highest mark.
Physics was one of my favourite subjects before I had her, and I was good at it, but after 6 months of her classes, I was regularly throwing up from stress before class. Which meant I got sent home, which meant I missed her classes again, which meant her bullying got worse.
Just to make it worse, at parents evening, she was utterly charming to my parents, and told them I was very good at the subject, no critisicm at all, so my parents believed she was a lovely woman that I just had it in for for some reason and ignored every complaint I made.
I nearly did take a bit of vengeance on her- several years later, she came into the zoo I worked at (which my family ran), with her husband and two kids, and I very, very nearly went up and told her in front of them all how crap a teacher she was, and how she’d ruined all the pleasure I had in the subject she taught. I thought better of it though; I doubt she would have cared, and unlike her, I didn’t want to upset kids for no reason.