"Would like to haves"
"Would be nice if I had, but I don't need"
"If you're willing to spend money, wish list"
That way you can back down from things and seem reasonable, but know what you MUST have in your space(s) to make it work.
Nearly every reputable school builder or architect these days will have someone who knows how to treat a band hall. It just depends on whether they're going to take your space seriously.
Make sure you know how much space they're talking about giving you. Ask about footprint. Just remember, every square foot of concrete you add to the foundation adds tens of thousands to the renovation's cost, so understand you WILL NOT get everything. Find ways to renovate existing space so that it works for you.
Also - ask to speak with the designer and/or architect SEPARATELY so that you don't take up time. On a committee, you'll be lucky to get 15-30 minutes of time, total, to talk to them. You need a good two to three hours to hash out details. Bold because this is probably the most important piece of advice.
Go see GOOD facilities near you and see what they have that works and what doesn't. Go see BAD facilities near you and see what doesn't work - and what may, with a little tweaking. And figure out how you want human flow to work.
Practice rooms: doors into the rehearsal room or hallway? Storage and office - same. Do you have prop storage, flag storage, and enough practice spaces? When one inevitably gets taken away and turned into an SRO office, network hub, or "junk closet" do you still have enough space?
Will there be LAN/electrical in the center of the room (for plugging in tuner/metronome/etc/chromecast/laptop) or only on the walls?
WORK SPACE. Get a large sink and a WATER FOUNTAIN in your classroom space. Seriously. Why send your oboes out and down the hall to get water? You need someplace to clean stuff too.
That's off the top of my head. Good luck.