Proms come in a few varieties... my school had 2, one in spring (for sophomores, juniors & seniors) and one in the winter, for juniors and seniors only. Homecoming I know squat about, as my school didn't really have one (I think it has something to do with football, and I went to a girls' school) and I wasn't asked to homecoming at any other school.
For my school, it was a little different, as it was a small school (my graduating class was 40 girls), so yea, you'd go in a group with your friends and their dates, but you might mingle a little more with everyone else.
I spent a semester at a much larger public school. I skipped prom, but that was a school where each graduating class was 150+ students. It was hard to KNOW everyone by more than a passing glance. (I'm trying to find out how that compares to the national average, but everything I am finding is giving class sizes as X students per 1 teacher (or how many per meeting, rather than the whole graduating class).) At any rate, if they have a prom for just juniors and seniors, figure about 300 people attending - a good chunk with dates from the same school, some skip entirely, some without dates, and some with dates from other schools. Or, way too many to spend any meaningful time with everybody, and a lot of them you don't know in the first place. So it's really not that people are being deliberately exclusive most of the time, it's just a HUGE number of people, and generally considered a night to remember with your friends.