Wiktionary
prom king
n. (context US English) In colleges and high schools, a young man, chosen by his peers, to "reign" over the school's prom and related activities. May reign alongside a prom queen.
Wikipedia
Prom King
Usage examples of "prom king".
So, though we calculated we'd elected John Reddy our Prom King by a wide majority, at least ninety ballots out of one-thirty, when it time for the announcement, at midnight of the prom, everybody excited, apprehensive, a kind of fever in us, looking for John Reddy (who'd been sighted since nine p.
They were the prom king and queen in '61, the year they graduated.