My 9th grade math teacher taught us very little math and would spend most of our class telling dirty jokes or anecdotes about his long life as a douchebag. After a few weeks someone told their parents about it, and the parents told the principal. He got a warning, and the principal's office started connecting over the intercom throughout the day to listen in on his classes and try to make sure he was staying on track. You could hear a click though when the intercom was switched on or off, and could faintly hear the background noise of the office as well, so he'd talk about math while it was on and go back to his usual routine once it clicked off.
Well, he was reported again and it made him furious. The office stepped up the intercom monitoring, often keeping it on all period. But anytime they did turn it off he'd rail about "that bitch" and threaten things like "your days are numbered". Eventually he convinced someone in the office to divulge the name of who reported him. Then he outed her to the class and said "this is the bitch who snitched and ruined things for us" and "someone should beat the shit out of her". After class that day, three girls ganged up on her in the hall, hitting her, pulling her hair, spitting on her. None of the other kids tried to stop them because we were all (myself included) little shits and/or cowards. The teacher heard the commotion and came out to enjoy the spectacle. Fortunately another teacher intervened and broke it up before the girl was seriously hurt.
The math teacher was fired, and a replacement came in and actually taught us math the rest of the year. The kids who beat the girl were expelled. Unfortunately the girl continued to get bullied by other kids who were mad about having to do math in math class, and before long she ended up transferring to another school.