I would be leery if the oil is clear. That means it just had an oil change which could be a innocent "help" from the church for the buyer or it could be covering up an issue such as coolant in the oil. Check the top of the dip stick under the cap for rust which might indicate coolant in the oil. Check the coolant overflow bottle. Does it smell like diesel fuel? That's not good. Not necessarily a deal breaker but some repair work.
Yes, if the bus is already warmed up it could be concealing some issues. Personally I would like to start the engine cold and then drive it for about 30 minutes (not just idle) to see how the temperature comes up, to uncover any hesitation in the engine (dirty injectors), overheating, etc.
Check the tires for uneven wear (alignment) and rock the steering to see if there's wear in the kingpins. Steering should be tight.
Oh, there's more but that covers some of the bases. Depends what engine/trans it has.
And if you're not mechanically inclined I would call the local school bus transportation office and see if there is a mechanic you could borrow to go look at it. If not, ask for the number of one of their veteran bus drivers that is mechanically inclined. Since it's summer break the drivers are just sitting around twiddling their thumbs and since most of them are retirees they're probably looking for something to do...
Driving... it's like driving a big long car with great visibility. Don't swing wide on turns but go beyond the turn and then cut. Practice in the church parking lot and watch where the rear tires are as you make a turn. And air brakes take a little getting used to. A well tuned system should put you through the windshield if you step pretty hard on the brakes. If it feels "mushy" ask them to show you how to check the slack adjusters. If they're good they can crawl under and adjust them for you which also may give an idea of how well it has been maintained.
