Ask for one offset wall (not 90° at the corner) to mitigate standing waves that happen in full band rehearsals.
If the walls are going to be cinderblock, have the voids filled with sand to reduce noise.
Have a dedicated repair/maintenance area with a big deep sink.
Consider carefully how HVAC will be incorporated (roof, outside the bandhall), and insist on a separate control so you can adjust the temps when school is not open but you are.
Download and read the Wenger info, and go with the other postings.