Fighting, even with bare hands, should be a last resort - whether 5-on-1 or 1-on-1. Remember, there are no teachers or referees to break it up if things go awry - and who's to say that just because you're out of the fight, they'll stop?
A few years of training would not prepare anyone to take on 5 attackers. It ain't easy to fight one guy who's an experienced fighter (I would assume that being a bully, he would have been in a few fights...), much less to fight more than one. The few (okay, two) people I've known that have succesfully fought multiple opponents (3+) in an open confrontation were:
1) Big guys themselves
2) Well-trained, experienced fighters
3) Highly skilled, exceptional fighters
4) In exceptional condition
5) Were tough as nails
6) Mad-dog mean when in a fight (both were pretty nice people other than that, though)
To fight more than one opponent, you have to be able to out-endure and out-fight them all. They all have to be exhausted, before you're even tired. This means, first, you'd better be in some serious cardiovascular condition - you're going to be doing a lot of moving around during this kind of fight (you never want to be caught in the middle), plus punching/kicking. Then, you'd better be able to eliminate each threat given the opportunity - so you'd better have skills, experience, & power to spare. But if you have all that, I don't think you'd be being picked on by bullies to begin with - they don't want to "pay the price" every time they want to intimidate you. So, unless you're some sort of marathon runner/triathelete turned pro boxer or something, HAUL YOUR BUTT OUTTA THERE!!!
None of this should be taken as discouraging taking some sort of training, however, just be realistic about your expectations. Nothing is going to quickly prepare (or ever fully prepare) him to face those kinds of odds. If you told me this kid was a 6'3", 260lb wrestling/boxing/karate champ, I'd still say running was his best option. One of those skilled fighters I mentioned gave me some sage advice: "If you fight, win or lose, you're gonna get hurt."
The only other advice I can give is be careful about what instructor you choose - that's probably more important than what style(s) you pick.
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Beginner barbarians probably had the idea that every house they broke into would be full of untouched loot and frightened, unarmed victims. It just doesn't work that way, my friend.
I hope these evil men come to understand our peaceful ways soon - My trigger finger is blistering!