You shouldn't have to rip out all of the roof, just a few of the panels above the windows. They're only a foot or so wide and shouldn't disrupt any insulation when removed one at a time. You need to just look immediately above the windows to see the welds. I haven't personally done this on my Carpenter body, rather it was done by the dealer, so I don't know for sure what the insulation is like, but there were very small bits of fiberglass on one of the seats as evidence that they indeed did pull one of the panels. It did not appear that insulation had fallen out, though.
Anyway, like others I think you have a nice bus with low miles. These things are trusted to carry our precious children, and as such are held to very high safety standards. The likelyhood of significant failure of welds is probably very low, but for the kids any possibility would be enough to create a panic. If you look at the number of recalls for passenger cars and trucks, it puts it all in perspective: skoolies are the safest form of transportation available :-)
So, it's probably a good idea to look at some of the welds, just to discover any significant problems, but if you pull one of the panels and find no cracked welds, then you should be okay.
I guess I should offer a legal disclaimer, as we are in the highest litigation-per-capita society in the universe: I am not a safety professional, and no guaranties as to the truth or accuracy of information provided are claimed. My comments are the sole opinion of myself and any injury incurred, physical or mental, is your own fault ;-)
Skoolies are the kooliest!