I was bullied extensively for my entire time at school.

Supposedly it's all changed now, and reporting bullies to teachers means they'll actually do something about it, but back when I was in school, the main lesson I learned is that bullies only stop if they actually fear the consequences of continuing. Since the teachers never impose any real consequences, I had to do it myself. The trouble is, you have to give an awful lot of bullies concussions to really get the problem to end, because it only works on the bully who is actually hurt due to their actions. It doesn't stop their friends until they, too, fear pain.

Interestingly, when I started A-levels at the local college, a bunch of the kids from my previous school were there as well. One of them decided to continue bullying me, so I did the thing I was supposed to do: report it. I was fully expecting nothing to be done, and it would follow the same routine as it always did: empty platitudes, promises to deal with it that ultimately went no where, ending in the inevitability of me snapping one day... However, the college had a rather different attitude to bullies than the school had. The first report was the last one, and the nasty little bitch got expelled that afternoon. Never saw her again. I hope getting expelled screwed up her life.

All the other kids from my previous school started being very, very nice to me. Turns out they can treat others with respect after all!

Children are assholes. Empathy is not innate. It's something they have to be taught, and if their parents can't be bothered to do that, they will be cruel, vicious little dicks who only care about themselves and who only respect others if there are meaningful consequences for not doing so.

The ideal outcome is that the school imposes those consequences before you have to take matters into your own hands. In some countries, if the school doesn't do anything, the police are a good option after that - in my country, the police don't like dealing with it, so they tend to come down like a ton of bricks on the school with major "why has this gone on so long that it has come to us, sort it out now" energy. Sometimes the school has to feel like fobbing you off is more effort than kicking the bullies out before they take it seriously.

I guess in short my advice is that to stop bullies, you've got to be willing to stand up for yourself. Bullies cannot be reasoned with, they can only be compelled to behave by the fear of what will happen if they don't. Sometimes the school also needs to fear the consequences of not taking reports seriously.