The best way to control them is to stop schools or associated groups from putting on any leaving event that requires a child to pay more than £5-£10 to attend or requires any clothes that a child would not normally have in their wardrobe - jeans and T shirt, for example.
When DD graduated from drama school, I think they all made their own dresses and DDs involved an old sheet and lots of paper hankies.
Actually, why not insist that all prom clothes, boys and girls, should be recycled from something else, like the second round on Sewing Bee?