Just a point about schools not endorsing 'after parties' it is a legal thing.
If a school doesn't make it clear, preferably in writing to the parents, that the 'after party' has nothing to do with the school and something goes wrong then the school is liable but by sending out letters, putting it in newsletters etc the school is making a legal distinction between the formal event that they have a say in organising and an 'after party' which hasn't the endorsement of the school.
I didn't understand why my boss starting sending out those letters about 10 years ago so I asked him and that is the reply. It is the same reason that many schools (if not most) don't actually give out information about 'schoolies' - so as there is no way the school can be held accountable it something goes wrong.
If a student is injured at the formal, for instance, the school's insurance covers it but if it happens at the 'after party' the school's insurance doesn't cover it as the school has made it clear that they don't have anything to do with the 'after party' - the organiser of that party is legally liable.