| At most schools, you'll need to audition not only on your instrument, but also present a composition portfolio of some sort to the composition faculty. At the time I decided to major in theory/composition, I was still a guitarist and the school I was going to actually had a guitar program. But I knew that the piano was really the greatest composing tool around, so I auditioned with piano as my principal instrument after a summer in the woodshed. At the time, I barely made it in, but I'm glad I did it that way because the piano skills I acquired over the next 7 years took me places I never could have gone otherwise, and were incredibly handy for theory/counterpoint/orchestration studies. Even if you find a place that lets you major in BG, I'd highly recommend building up your piano chops while you're in school.
All instrumental questions aside, the decision to major in composition is not one that should be taken lightly - you have to LOVE writing music to do it, because you'll be required to write every week, and there's almost nothing worse than trying to FORCE yourself to write week after week. I highly advise seeking out someone who has been down the road you're contemplating before and picking their brain about what the experience was like... if possible, someone from the very school you are thinking of applying to. You may find out that it sounds like a great experience, or you may find that it sounds like hell. Either way, it won't hurt. Good luck. | 
