It would help if topics like these are in the open.
I understand it's not easy to speak out but as a parent you should know your child. A teacher should too. Children spent at least 8 hours a day at school.
An abused child changes, there are plenty of signs that should let an alarm go off.

I asked my child daily after school: how was your day? The answer was: fine.
Till the answer was: fine I even had a good time.
That was a huge sign to me! I am sure all those teachers still remember me teaching them a lesson.

My biggest fear is that today's small bully is tomorrow's rapist and serial killer. Bullying is not just joking around. ☹️