i was going to say bake sales but you've already thought of that, hmm raffles, sponsored events - what about a talent show or something like that - our school did a takeoff of i'm a celebrity where the staff and students ate some disgusting stuff for charity, to watch it in the main hall you had to pay at least 50p but pretty much the whole school came to see it so they made A LOT of cash, i dont know how these work but set up at online fundraising/ maybe gofundme i've heard of but make it for your year groups prom and then advertise the link on facebook so people in your year can donate? other ideas: sponsored school disco, sponsored silence, man VS food - pay entry to watch
could you get a prom committee together which could find a good / cheaper venue? if the school is making you pay £££ for it then you could form another prom committee with just you and your friends and involve the people you've associated with and invite people you know and find somewhere else which hopefully doens't make you completely broke.
have a good think about ideas because charity events are coming up like children in need so people will already be fundraising for that as well as non uniform days and christmas jumper day nearer christmas as well