Here goes, for a "rust" state I don't see anything particularly glaring. Bear in mind I grew up and live in one of those states and it's not unusual to see a car or truck rusted out up to the windows, then more rust above the windows.
I'll mention a few I do see.
23 if that doesn't go through it soon will. Proper fix would be take out window, then weld in both overlapping pieces.
26 Can't say for sure, probably rust clear through.
27 Note under hinge is clear through, hinge area is probably weak.
31 If this is underside of driver floor doesn't look real bad. On a car or truck this area is quite prone to rust since driver gets in and stuff drips of of boots as well as getting from underside, not so much on a bus.
33/34 This appears to be outer body brace behind right rear wheel. As you can see that black tarry undercoating tends to hide things. I hate that stuff, water will get underneath it and rust the metal but not be visible. It either falls off because of the rust or is really nasty to remove. Looks like picture taker might have just flaked it off in the picture.
35-38 Not too bad, but will need attention, if you're a welder just cut out bad and weld in new, most, I assume will be covered up in the end. Area towards front is probably in not clear through. Rug rats with their salty boots(doe's any kid wear boots anymore?) track in all kind of nastiness here.
39 That looks nasty, can't believe state\school safety inspectors let this go. At very least should have a coat of paint before it gets this way.
40 On closer inspection this on concerns me. This is I think left rear frame, not how the paint looks to have large humps in it at the right side? There is serious rust under that paint. If you poke at it, large flaky rust will fall away. The frame will be thinner at that point, Can't say how much, I'm just pointing it out. At the very least you would have to chip/scrape/sandblast and paint.I don't know what kind of inspections you have this may be a problem.
42 Same as left frame, None of the tires have all that much tread.