When I was in high school, there were certainly cliques like that, but nothing nearly as overt as is shown in movies. I had friends in all different groups.. I like cars so I got along well with the rednecks, computers so I had a lot of geek friends, and I had a few jock friends too which meant I never really got picked on.
Doing other peoples' homework isn't extremely common. A friend and I would usually do half of the assignments and then copy each others' stuff (we'd alternate halves since the first problems were generally a lot easier too), but we were more clever than average.
I've actually gotten far more discriminating in who I associate myself with since high school. I used to try and make everybody happy so I had a wide circle of acquaintances. I'm much happier with being myself even though most people think I'm an asshole, and having a couple close friends.