Just say you don't like dances much, and you'd like to get to know her better, and you can't really do that with loud music playing, and other people bothering you anyway.
Also, if it is a public school, I bet that the school is just ripping students off their money anyway. Seriously, my school's homecoming tickets are $10, and at least half the school ends up going, resulting in $10000+. The teachers (if they even get payed for standing and watching around the barely decorated gym) cost $1000 at most, and I doubt that the DJ, and cardboard cutouts cost $9000+. In-fact, the rule with dances is that 200+ (I think, or it may be 400) students have to attend, so I'm assuming that is it all profit from there, if not before. Heck, my school even changed $1 for a can of pop.
