You could always use rumor and innuendo - without any of it getting traced back to you as the source, spread a little dirt on each of the girls. Do it in such a way that each girl will think another girl in their posse is ratting 'em out. Use bathroom walls, the internet, graffiti, etc. Since they'll be accusing each other of being the snitch, they'll be too busy to hassle you.
If they're participants in the talent show, sabotage their act somehow. Pay off the sound guy to throw some feedback and distortion into their act, especially if they have a soloist.
Otherwise, call up your local Mafia don, and ask them for a favor. Always come to them as a friend (invite their family out to dinner on you, bake them some of those delish Italian cookies, etc.), and show them lots and lots of respect. Don't ask them about their business, though. Do this, and your enemies will become their enemies. Then they (your enemies) will fear you.