Yes, your assignment is to teach band. But if ONE rehearsal lost in the middle of April will wreck your concert/UIL/performance, you're doing it wrong or you simply overprogramed. What if you got sick, or in a car accident? Will UIL just not happen then? Will you just not even try? Will your kids give up without you? Will the concert be cancelled? Probably not.
Support your admin and your campus in this, that way, next time you ask for something, they remember you being a team player and not just someone who didn't want to do testing.
Testing sucks for literally everyone - kids, teachers, aides, hall monitors, and administrators. Because of the short-sightedness of Mike Morath, all of these tests still count and will determine graduation or promotion status for our kids. You can get on board and help support the school, or you can stay in your room and hope that everything goes OK without you. I know which one the administration is looking for when it comes time for "extra duties."
And do more things to reduce your bus number. That's an IT term, but it's absolutely the truth - if you were incapacitated by a bus tomorrow, would your band be able to function? Or is it 100% dependent upon you to make band work?
Give your kids sectional activities that can be overseen by a sub. Give them small group assignments. Have them play measures onto Flipgrid or in SmartMusic. Think outside the traditional band box, as it were.