Which do you think I should do??

My friends are playing in an orchestra concert at my school 2morrow night. I want to play with them, because they need a flute, (It is a trio...Quartet if I'm there) and I already told them that I would be there. I got out of my 1st and fourth period today practicing and learning the music we would be playing. BUT: I really like this guy that works at my local movie theater, and I had plans to go to the movies with my friend so we could see him. I don't know when the next time I'm gonna be able to see him, or do something with my friend is. I really, really want to go see this guy, but I don't want to lat my other friends down because i already said that i would be there.....But i also told my other friend that i would go with her to the movies. Which should I do??