If you're looking to save cash do this:
go to restaurant.com
and enter your zip code.
Find a suitable restaurant (Like Il Lupo in Fairfax, which would be a really good choice I think).
When purchasing your dining certificate enter
Promotional Code: 66289
That code is good through 5/1/05 for 50% off.
Hence, you can get a $25 meal certificate for $5.
Many restaurants won't let you use it on alchohol, but you're underage anyways.
Be sure to check the restrictions. Like they might not let you use more than one certificate per table. Plus you'd have to guage socially whether your date would think you a cheapass for using a certificate on prom night. But if she doesn't go for it, maybe you should be rethinking the relationship anyways. Unless you're not thinking long-term, just trying to get laid or whatnot.