Oh and I forgot to mention that it's a boarding school (for poor kids

) and so the people are all in my 'house'.
Easy there chap.
Boarding school, you say? Do the kids have a dorm to themselves, or do they share?
'Cause if they have a room on their own, the next guy who dishes out a pasting to you, let it slide, then a couple of weeks later pay him a visit in the middle of the night and beat him out of sleep with a hockey stick. And tell him the next time he tries it on, he'll be waking up on fire, not just to a few bruises.
Obviously you don't actually hurt him much, or really set him on fire, but the psychological fear you instil ought to at least give you a break. If it was like my school, they won't grass you up - that's not how it works.
They might be hard in a group, but catch 'em on their own and vulnerable... You might get labelled as a bit of a nutter, but you'll get left alone because they'll be afraid of the consequences if they don't.
The battle is lost or won in your head. And in most cases there's little point in a straight up fight if the other guy and his mates are bigger than you. So you have to use your cunning and be a little bit eye rolling mad
I found this technique has its risks, but the desired effect was satisfactory in its long term goal. I speak as a former 12-18yo boarder myself.
Your results may vary, but the principal is sound - catch them alone and vulnerable and get some beats in, no preamble: 'if you're gonna shoot, shoot, don't talk' as it were.
If all of that is a bit to near the knuckle and deliberate, then your reaction has to be immediate when the bullying is taking place, preferably when the other guy is working himself up by giving it loads of mouth - he's expecting you to be intimidated by this so he can act all alpha in front of his mates, wipe the smirk off his face with your elbow in his teeth and when he goes down, or leaves his body exposed as he reaches for his mouth, follow it up with the palm of your hand or your knee, smartly applied to the fork of his legs.
It's not a nice lesson to administer, and you might take a beating anyway, but most bully's are cowards and will think twice before risking loose teeth and bruised pride again. Plus you have the knowledge that you didn't just stand there and take it like the last time.
All of this stuff about ignoring them and they'll go away is defeatist clap-trap. You have to speak their language: fear and violence, I'm afraid.