It's...complicated. This is one of those Deep South things where after Brown v. Board schools couldn't formally host segregated proms, so they stopped hosting proms entirely and it became a purely student/parent organized affair (at least in some places). There would be a white prom and then a black prom for the high school. And this is legal because it's a private party. You can do whatever you want for a private party, including making it racially segregated. There's a documentary narrated by Morgan Freeman called Prom Night in Mississippi which is about the first integrated prom at a small Mississippi high school. It's a genuinely fascinating topic in part because of how it makes you think about the fundamental function of prom. I mean, why do proms even exist, what are their functional purposes, cultural antecedents, etc? A lot of how they operate and their purpose is tied to the history of debutante balls, which was a way of "presenting" women into society (which is to say they're ready to be married off in order to form deeper political and economic ties between wealthy families) but they've morphed into a signifier of a sexual "coming of age" of American teenagers. That's why so many movies have teenage characters trying to lose their virginity by prom, or even on prom night itself. In conclusion, prom represents a confluence of racial, gender, and class signifiers (and the innate prejudice associated with those who don't fit the expected mold for those things).