Teachers are often the main people to blame for bullying. During my school years, it was often the kid that received the most flack from teachers that got bullied the most - next to the kid that kissed teacher ass, of course.
But during my high school years, I got bullied for a few years too because I was "different" as in, I dared play computer games *gasp* and liked reading and scifi whereas they liked getting drunk and having one-night stands with slutty girls. They started with small stuff but soon, not a day went by or my lunch got stolen or wrecked, books stolen, other property stolen or wrecked.
One time, during the class of one of my favorite teachers (who, unlike most others, didn't take bullying kindly), they stole the case of my brand new glasses. Inside was an expensive watch too, which I had received as a gift a week or so earlier. The case then got thrown back without the watch. I yelled back that they had 5 seconds to return it. They just laughed - it was the last drop.
I knew the three guys who would be responsible and got up and ran over to them and grabbed anything I could off their desk and threw it out of the window (we were on the 3rd floor) including a calculator, all sorts of pens, one of the earliest mobile phones too (this was 1997 when they were just appearing). They were scared as hell, went "OMG you're crazy! You're crazy!" (yes morons always think you're crazy when they bully you for ages and then are surprised they push it too far and you flip out). One of them threw the watch at me to get me to calm down, but they had already broken the clasp at the back so it couldn't be worn any more. When it was all done, I said I'd send them the bill for the watch and they better pay it or I'd break everything of theirs until it matched the bill.
During this whole time, they looked at the teacher as if to say "when are you going to step in?" and he just stood there watching. When everything was over, he just continued with the class. After the class, as I was leaving, he stopped me and said "You better not do this in other classes, those teacher may not get what bullying can do to someone. Don't let them get to you too much, okay?"
Now THAT was a good teacher. He really disliked bullies himself but later explained that teachers often make matters worse by intervening and punishing both because then the bullies feel the blame is spread to both.
I did have my own ways of getting back at bullies - I got several thrown out of school for getting back at them and they deserved it.