It seems to be a thing people do when they are teenagers - typically - a sort of dance that marks a celebratory passing of the educational year/cycle.
People take dates, and there is dancing, usually chaperoned, perhaps followed by less chaperoned activities elsewhere.
I haven't sorted if there will be alcohol at this one or not, but seeing as how it seems to be for everyone - I'm wondering if there won't be, even if it is only brought by people attending. Typically there wouldn't be as they do not allow people to drink alcohol here until they are ridiculously old.
I've been trying to decide if it's worth seeing if Skywalker wants to go.