Unfortunately, in some of these circumstances, the parents can't tell the difference between a "tough teaching style" and abuse.

Which is how so many kids end up being abused/bullied by sports coaches. I put up with a terrible horseback riding instructor for my entire childhood because my parents didn't have time to take me to a barn further away, and I love my parents, but they never saw the truth about what she was doing. (In addition to being emotionally abusive towards the students she was physically abusive towards the animals, which is a major red flag. She'd probably have hit kids if she thought she could get away with it).