How big was your high school? I hear this saying a lot but I genuinely do not believe that some 200+ students per grade all intermingled and did not have any social groups at all.
High school can be an absolute killerfor some kids because of how dumb teenage minds work. That said though, people break off into social groups for necessity and often to compliment personality especially in that age bracket. I find it hard to believe it when people propose that everyone was friends with everyone.
I find it more in the realm of believable if there just wasn't any active bullying or asshole behaviours towards your peers.