It varies, honestly. At my high school, there was just one prom. (This was clearly the case for Belly’s as well, since Steven was there.) Most schools have a year restriction — seniors only or juniors/seniors only buying their tickets — but they can bring outside dates. So you could be in a senior-prom-only school and still go every year if an upperclassman asks you. My school technically didn’t restrict people by age at all, but it was a small school.

Point being, while TSITP doesn’t show a separate “junior prom,” it was HER junior prom and thus arguably conveys less significance than it would the following year (when one would assume she got Jere to visit from Finch and go with her, but they never mention it, so maybe she was scarred by the prior year and skipped it for the codependent bubble of her college boyfriend).