I used to fret over taking my handgun (usually a revolver) into a hotel and having to leave it there. I got tired of worrying about it and changed to taking an autoloader that I can take apart easily and quickly. When I have to leave the room for the day, I take the thing apart and throw the barrel in my briefcase or a jacket pocket, take that with me, and leave the frame and slide separated in the room safe or locked in my luggage. If anyone gets in the safe, they aren't going to hurt me or anyone else with the pieces and parts.

In the situation you seem to describe, I would take the gun along in the car and leave it there in a locked briefcase when you go into the school building - as long as the inside of the car doesn't get too hot. If it will, I'd just leave it in the hotel.