Okay, this describes pretty much every teen-focused television show, like, ever. No one on “90210” (OG and remake versions!) socialized with anyone aside from rando guest stars who’d end up in jail or switching schools within three episodes.
That said, I happened to go to high school in a similarly small-town-ish environment (albeit in bumfuck Texas, not Cape Cod or whatever area was being faked on the show), and my graduating class had exactly 100 people in it. While yes, everybody basically knew everyone else — and everybody used to party with everyone else, since there was usually only one cool party happening on any given weekend — kids otherwise socialized in cliques roughly comprised of 6–8 people, all of whom fit one of the basic stereotypes seen in any ’80s John Hughes movie: jocks (with subgroups based on sport(s) played), stoners, druggies (not the same as “stoners” — long story), rich kids/preppies, nerds, dorks (not the same as “nerds” — long story), band geeks, and theater kids (basically “Glee” if it consisted entirely of Rachels and Kurts).