Bullies - I guess not. Jimmy doesn't tend to bully people unless you make him. It can be argued he could fit in due to his friendship with Russell and the clique's loyalty throughout the game but I still don't see him as a true part of this gang. Either way I find all of them generic and non-interesting aside from Russell.
Preps - Well, Jimmy's mom was probably relatively well off considering the amount of rich husbands she had and the fact that she was able to afford a cruise ship holiday for a whole year with her new one. Jimmy has a kind of boxing style when he fights which is what the preps use. Maybe Jimmy could possibly be considered a prep?
Greasers - I don't think so. I can't think really think of anything he has in common with them.
Nerds - Definitely not. He doesn't have any nerdy interests and is way too tough.
Jocks - Jimmy doesn't seem to have too much of an interest in sports and is way smarter. I can't see him being a part of them.
Dropouts - Maybe in the future Jimmy will drop out of school. Then I guess he could be considered part of their gang
Non-clique: This is the most obvious one since he has the same fighting style and isn't officially part of any clique. It would be an insult to lump him in with these guys though since they're the weakest in the game and he's pretty much the toughest. They're a boring clique which fades into the background half the time and are only there to be bullied by the actual cliques
So, in my opinion, if we're not counting non-clique, I'm gonna have to go with Prep