We call them formals over here and my experience with them is confusing.
Our school has 2, one in year 10 and a final one when you finish school. The one in tenth year was cancelled because laws changed and you couldn't leave school early anymore, so they figured "Hey, no-one's leaving, why bother?"
Fair enough. But we were the only year that happened to. Next year and the year after got their formal.
That, and a girl attempted to create a student funded one for us in protest, and after acquiring at least $2000, she suddenly couldn't do it. Because laws, or something. No-one ever found out what happened to the cash.