If it's a public school, then you're really donating money to the municipality, even though it may be restricted to use by the marching band. This should qualify as charitable without any problem.
If it's a private school, it is more than likely charitable as well. Basically, if you can make donations to the school and get a deduction, then the donation to the band is really just a restricted gift to the school. Most private schools send out mailings asking for money and they almost always say if they're 501(c)(3) on those mailings. Also, if it's a private school, it should be listed in the IRS database of approved charities - publication 78 - which can be
accessed online.