Only our special needs and events buses had air conditioning. I usually drove a special needs route, but the AC never blew on me, so while the kids did okay, I was soaked.
I wanted to open the window, but I did not want the students' cool air blowing out my window.
This is according to the EPA. The worst idler uses less than a gallon an hour.
While I drove on days that were 110° or even 115°, opening the windows hardly did any good when the bus was parked. Whatever wind there might have been blew over my head.
I tried to not run the bus just for air conditioning, it seemed like a ridiculous waste, but the alternatives were waiting in the sun or hanging out in a solar oven. However, we often had four buses waiting together, and at least we could turn off three of them.