Reading through the responses you've got so far has been quite interesting. Plenty of good advice, and some I don't agree with. I'd also point out that gender does factor in, since kids usually form their groups around gender and those groups seem to operate under different sets of rules. Everything I say is from my experience with the male dominant groups, as well as assuming we aren't talking about bullying that includes physical violence.

I'd advice not to throw a tantrum or completely ignore them either. Former is exactly what bullies want, and latter allows them to run the show. I saw someone advice a surprising reaction, and I agree with that approach, as it messes their script. But it has to be surprising in a way that doesn't imply they got to you. One could try laughing with them or agreeing with something negative they said about you and compliment them in turn (when it can be done truthfully). Claiming that they are boring you also sounds like an interesting method, but I haven't tested that one myself.

I've personally also found being curious to be an effective tactic, especially around your age when the kids in general start to question things. For me curiously asking about their motivations for their behavior ("Why do you think you feel the need to bully others?") and such often resulted in them either starting to question their behavior themselves, or avoiding me to avoid the questions. I suppose they didn't like being treated like a curiosity or a strange research subject very appealing either. One could also use insecurities of the bullies in one's advantage with this method, as those are usually easy enough to figure out... but that could backfire should one be too confrontational.

Something I'd caution against is direct confrontation, but it can sometimes work if you're sure you can win that exchange and the ones that will follow. If not, it might just make the situation worse as a win like that is a gourmet meal to bullies and they'll want more.

Unfortunately, the things you or any outsiders like teachers (if they bother to try) can do to stop bullying is quite limited, and very much dependent on your exact situation. Most effective method by far seems to be the group in which the bullies are in (or one they respect) to turn against them, disapproving their behavior or starting to exclude them when they do bully. That probably won't help you right now, but keep that in mind if you end up part of a group where someone tries to elevate themselves by bullying. Then you might get a chance to save someone else from being bullied.