If you donated it to the band, with the instructions that it was for a specific student, then it's not a donation. It's a gift and not tax deductible. If you donated it to the band/school and allow them to do as they please with it, then no problem as a deduction. If you donated it to the band boosters (which is a separate organization from the school) and the band boosters is a registered 501(c)3 organization (most are, but not all) then it's a deductible donation.
When you mentioned a receipt earlier, I assumed you were talking about the receipt you received when you originally purchased the instrument. Use the donation value on the receipt given to you by the recipient of the donation, and you'll be fine.
