36" doors for EVERY DOOR, at the minimum. Double 36" doors with no middle bar if you are going to use bigger keyboard instruments.
Double the space for a music library than you think.
Double the storage you think.
DO NOT let them put their own choice of built in sound or recording equipment. Choose your own.
Some form of smart TV or projection system. Do not let them choose the computer they hook up to it either.
Space for lockers in the room you are planning on putting the lockers in.
I've seen some bands that will use a roll-up door instead of standard doors for entrance to the loading area to help move big equipment.
Power and audio plugs in the floor near where you're going to put the podium.
SOUND DAMPENING. Don't let them skip this.
Also, don't do like a friend of mine did and forget to include uniform storage room. And if you have a guard/auxiliary storage.