Prom dress

I saw my lovely neighbour piling the kids into her car this morning. It's half-term, so I asked "Going somewhere nice?" She replied: "Going to buy a prom dress for Clare." Clare is her daughter, and will be sixteen on June 4.
Prom dress?? When did this transatlantic custom arrive? I don't begrudge Clare her dress and a nice night out one iota, but, man, I don't know why we don't just transmute the "special relationship" into an application to become the fifty-second state of the USA.