You don’t and can’t know what the school needs. The school could easily need something completely out of your control, e.g. they need a 10th grade girl day student and you are a 9th grade boy boarder. You might be 10x better than the girl day student they accept, and it just won’t matter.
My kids weren’t musical, so I don’t speak from knowledge, but I don’t believe most boarding schools have “band”. They seem to have orchestras and various ensembles. Perhaps someone who knows more can chime in.