Emo kids would be fun, but they'd make the game date a lot. My personal theory is that "emo" is going to fade into obscurity in eighteen months, and the emo kids of today will either find something else shiny to dress/act like, or they'll grow up a bit and fall into the goth/alternative scene.
That said? My clique ideas?
Can't resist the emo kids. The cliches would be all sorts of funny.
Goths: and their rivalry could be with the emo kids. I assume they'd tolerate the nerds (maybe even roleplay with them), and pretty much be ignored by everyone else, probably harrassed by the bullies and the jocks.
Paris Hilton wannabes: This could be a predominantly girly clique, full of girls who say "that's hot" and stuff all the time. Maybe there'd be a token guy as there seems to be a token girl with all the other cliques.
The wiggas idea is hilarious. *grins*
Surfies: Given there's a beach in Bullworth, I wondered where the surfies were. They'd hang out, go surfing, bum around and get Jimmy to acquire certain "secret packages" for them, I imagine. The Preps would probably loathe them, and there'd probably be rivalry between the Greasers and them.
Hippies/Activist types: semi-political kids with little sense of humour and a heart full of rage and activism- Lisa Simpson types but probably not quite as bright, and more inclined to want to fit in. They'd probably loathe the Jocks, empathise a little with the Townies and the Nerds, want to save the Bullies, and be mystified about the Greasers.
Fundamentalist Nutballs: Sure, they wouldn't get included in the game for fear of offending people- but they'd be hilarious. Basically, they'd walk around quoting (and messing up/taking out of context) religious texts, complaining about the lack of morals in the world, dissing kids for kissing (particularly if they were both of the same gender!), dissing the Nerds for RPing, dissing the Preps for being rich, dissing the Bullies for violence, and trying to convert everyone else.