DO NOT BRING A WEAPON.
No offense to those of you giving that advice, but THINK for a moment. Those kids pride themselves on their reputation of beating people up, if you bring a weapon, and thrash one or two, or maybe all of them, what's going to happen? They'll get bigger weapons, and injure/kill you.
Bringing weapons to a fight is a bad path, because it escalates, and eventually, someone is killed.
Bullies do what they do for a myriad of reasons. They can do it because they like the rush of power over dominating people, peer pressure from friends to pick on people, some may honestly enjoy making people sad, others are trying to maintain a social structure, others might find they are more popular, (though in a bad way), others might have been beaten at home and think it's alright.... I could keep listing.
How/why doesn't matter. What matters is how you deal with it.
Go to the police, or if you want to start smaller, go to your principle. Travel with a group of friends, and TELL your parents! That's important, they can't help if they don't know..
I know you heard this all before, but in most cases it does work.