These videos should show you that your conception of how a bus body is built is pretty different from how it's actually built and that your idea of how to raise it won't work very well. This video shows your specific make of bus being built (not in great detail but enough to see how the structure is formed):
And these two videos go into more detail about how an International bus is built - the construction techniques are nearly identical:
The ribs of a school bus are not directly attached to the floor in any way. They are attached via tabs and machine screws to the chair rail structure which is itself screwed to the floor; the multiple-bend shape of the chair rail not only secures the outer sides of the seats but also helps ensure that the ribs do not bend inwards as the result of a side impact. The side panels are riveted to the ribs with the skirt portion riveted to triangular brackets screwed to the underside of the body floor.
You might conceivably be able to grind out the screws that attach the chair rails to the floor and remove the rivets that attach the skirts to the underfloor brackets and then lift everything, but you would then have to build a fairly substantial steel-framed wall (equal in height to the height of your raise) to span the created gap between the floor and the bottom of the raised chair rail. The weight of what you're raising would also be substantially greater (probably about 2.5X) than with a traditional raise.
I suppose this would be doable, but definitely not simpler than the standard methods which have been used by many people. If you're an experienced metal fabricator with excellent welding skills, it would probably be fun to watch this being done. If you've never done anything like this before, I think your odds of success would be a lot lower than with a traditional raise.