Here are photos from the rear of a model year 2000 Blue Bird with a rear Cummins 8.3L engine with the access door into the cabin visible. The rear bench concealed it on the inside. This one isn't a skoolie, but I assume the school bodies would be similar.
As I understand it, this engine is designed for in-frame rebuilding which would include replacing the cylinder liners (replace the bore instead of machining it). They're about the size of a #10 food can, aka coffee can to some. Certainly the access would be invaluable for that job. I've replaced one fuel injector line back in that area and had no need to open the access. Haven't done a valve adjustment yet, and although it might be easier for the back few cylinders, I think there'll be room to get the valve cover off and do the job without opening that access. Other engines (I'm thinking the 5.9L) may not be designed for in-frame service and it's anybody's guess whether those bodies would still have the access door.
I think you'll do fine to assume that it's remotely possible, but quite unlikely, that this access door would be needed during your ownership. If the work to be done were that major you'd be living in a hotel while it was done. Or you might just sell it for scrap and get another bus.
The State of Utah is considering legislation that would result in districts here retiring about 170 year 2002-and-older buses over the next few years. If Utah isn't too far for you to travel, you could tune in to that process and learn where and when their inventory will go. Far and away the majority of school buses here are flat-nose.