How do I keep a great party from turning sour?

I'm graduating from a private girls school (yay! graduation!) and it's my job to throw a formal farewell party (because I'm student council president). We planned a really cool party with live music, 6 chocolate moose pies and this GIANT silky cake. I even bought a $4,759 red, strapless dress just for the occasion. But now they say we have to allow each senior to bring up to four guests. Some of them are inviting people from really gross parts areas (and I know because I have the RSVP list). I thought this was going to be an EXCLUSIVE party so now I'm totally disappointed. Should I save the dress for another occasion or waist it on this lame party? Argh. Or should I go to the principal and fight this? Maybe I can convince her to let student council trim the guest list to people we approve of? This sucks.