Fin,
There a good schools and bad schools. Fact of life.
I don't want to say this, but whatever happens, his future at the current school could well be difficult for him. Being studious and good at piano, karate etc. makes him a natural target for the no-hopers and other scumbags. It's a good case against comprehensive education, I'm afraid.
I would start looking at alternative schools. If he is as talented at the piano as you say (and I am sure he is), consider a school that specialises in music. My own comprehensive school down here does (partial selection on music ability) and there is also the private Purcell School in Watford which specialises in music. Obviously, these are of no use to you, but serve as examples. I suspect that a little trawling around will find similar opportunities up there.
I suspect, if you handle the situation well and are a bit lucky too, little Fin will not get a criminal record. Being attacked by 20 entitles anyone to use self-defence, I would have thought. However, it is, in my opinion, VITAL that you get witnesses prepared to stand up and be counted. BTW, does the school have CCTV? We do in ours and it's a good system that records to HDD and archives for at least a week.
Good luck mate!

...and keep us posted.