Me neither, but it's a woman. This happens all the time.
I chuckled.
Problem is, my best friend that lives in another city doesn't think she can get a date for prom in time and wants me to go to two proms. Which is technically feasible as they are on May 1st and May 9th. However, the prom tickets are expensive. 70$ for both and I can only afford one. I've already asked the drama class girl, but still I can't have my best friend go alone.
Pick the one you might end up in bed with. That's what guys are supposed to sound like, right?
Actually, this is another instance in which being straightforward and uncomplicated makes the most sense. Tell your friend that you've already made a commitment (or at least proffered one) and that you can't afford to serve as her stand-in date due to said commitment. If your drama class girl declines, you'll be free, which will be great, but as it stands you've given your offered word to someone else. With this course, you're being honest and not doing anything unkind to anyone. It's perfectly appropriate and honorable.
Or, you could take the ultimate good guy route and sacrifice your own prom and romantic interests to quell your friend's loneliness (and if you retract your prom invite to the drama girl, it may damage your chances with her later). There are very few people, though, who can make a decision like that without suffering regret. If you choose this option, there will be little in the way of personal fulfillment beyond the cliched "I know I was the good guy" bit. =/
I'd recommend the first paragraph.