My son was on the receiving end of some bullying last year. Unfortunately nobody at the school believed him because the kid who was doing it was physically smaller than DS--but he was verbally and emotionally just nasty to my DS (who may be big but is very sensitive), and nobody seemed to care until the day DS wound up and socked the kid. DS had not said anything to us about the situation I guess because boys want to seem tough and who wants to go crying to mommy...I dunno. Anyway, I hadn't been aware of all this until I got the call that DS had socked the other kid, and I asked DS about it and he told me the whole story (which had been going on for weeks)
DH and I sat down with him and expressed pride that he had not been the aggressor, and that he had been so patient...we reminded him that it's better to run away or find an adult etc, rather than to get physical, but we didn't get him in any trouble for defending himself. I think that's really important--to make sure that your DS knows that you are not mad at him for defending himself, esp if this has been ongoing--but of course then it's also important to try to help him find better ways of dealing with the issue.
I agree with the pp who said to lay it out for the school. Where is this bullying happening? Who is supposed to be supervising at that point? Tell the teacher, the lunch monitor, the recess monitor, the principal, or all of the above. If they've all heard already, and still nothing is being done, then I would call the superintendent AND also file a written complaint with the school and the district. Having it in writing will be beneficial if you have to take legal steps down the road. Obviously hopefully it won't come to that, BUT putting it in writing also shows everybody that you are dead serious about this.
I don't have personal experience with martial arts (it's not available to us at present) but I too have heard good things--giving your son confidence will certainly be a good thing, and I don't see how it can hurt for him to have the skills to stand up physically too (it might make some of those teasers think twice before harrassing him and he wouldn't even have to do anything, kwim?).