Otherwise, we'd have to apply NPNP to little league baseball (since baseball is governed by UIL in Texas), club cheerleading, and everything else. This isn't NPNP territory. This is wholly separate.
The only thing that may be getting you in trouble here is that the money is being passed through the booster club. If your boosters are incorporated as a 501(c)3 organization then you're likely in the clear. If, like most boosters, they are simply DBA a charity, they're technically an arm of the school. Regardless, this is not a "band" event, it's an invitational.
Folks, ask yourself this. Do your athletes have to still be eligible to be nominated for "Athlete of the Year" or for "Wendy's Athlete of the Month" or "Division 18-7A Newcomer of the Year?" Do your kids have to be eligible to audition for a scholarship? The answer to both is no. This is the exact same thing. NPNP does not apply.