Tell people that you're worried about being attacked, bring a portable noncombatant witness around with you whenever possible (preferably a cute and harmless looking one who can wrap people around their middle finger). I personally disagree with the doctrine of 'telling the school' unless you have a good relationship with them, because I for one have neither seen nor heard of anything remotely resembling rational behavior from any school regarding aggression or violence. Tell your parents, though.
If he talks ****, just tell him you don't want any trouble and try to walk away.Try to also point out that it would be cowardly for him to attack you with a bunch of friends. What's that supposed to prove?
If he -actually takes a swing at you-, reel back from the punch, ham it up so that anyone and everyone in the audience can clearly see that you were just attacked without provocation and took a hit, then proceed to f!!k him up HARD in the most humiliating way possible. Don't forget to not deviate from your training - MA's get f*ed up when they start changing their game plan. Beginning capoeira regional players have been known to trounce muscled street fighters when they stick to their form; expert fighters have been flattenned when the striker goes for a clinch or grapple, the grappler tries to box, or whatever.
Leave precisely zero doubt in the mind of anyone watching that you are completely capable of repeating the feat on anyone in the audience on a moment's notice if forced to.
Then just leave. As I have mentioned, schools rarely have any sort of sane responces to any form of fighting in their walls. (if I sound pessimistic, it's for a reason.) If they track you down (or bother to), then that's what your portable witness was for, have them smile and pour on the stuff about how you did everything you could to get away and how you were cornered and attacked with no provocation. Mind, if you're friends with the people in the office, then a lot of the stuff about avoiding dealing with them might be misguided. I leave it to your discretion. I had a counselor help me quite a bit when I was attacked and beat horribly once. In my experience, though, this is more the exception than the rule. The danger is that they will make the common (if bizzarre) assumption that because you do a bit of MA, you should be able to defeat him by lightly touching him on the wrist and anything more constitutes lethal force.
Schools at the HS level or below generally seem, in my experience, to utterly ignore anything below the level of a raging gun battle as 'kids will be kids', but occasionally, based on administrator PMS or how bad a day some secretary has had or some other totally non-predictable reason, will go absolutely totalitarian on your @$$ and start throwing around suspensions at anyone who was -in line of sight- at the time, with no perceptible policies.