600+K miles on a school bus, particularly an eastern built bus, is going to be pretty tired. Even if the engine and transmission had been rebuilt/replaced along the way, but as Tango wrote earlier everything else would be pretty tired.
Back in the day when Crown and Gillig offered frame rails of 150,000 PSI steel a school bus with 600K miles would still be in pretty decent shape if proper maintenance had been done on it. I have seen many CA built buses with close to a million miles that still were in good shape and could still get down the road and back without any problems.
But today the "heavy duty" buses have 50,000 PSI steel. I drove a lot of miles on Crown and Gillig buses before I drove a brand new BB AA RE 14-row with a 285 HP Cat 3208T in it. I had the varsity football team on the bus with the luggage compartments crammed full of their junk and plunder. In other words I was loaded about as heavy as the bus would ever get. I had to stop at the top of the driveway leaving the school parking lot. With our old buses we were told not to stop so we wouldn't have to slip the clutch to get going again. When I went to go I mashed the throttle. I could literally feel, see, and hear the body as it twisted as we got started. It was not a sensation I have ever had in a Crown or Gillig.
That story is just to let you know that a BB with that many miles would be pretty well worn out. The CS models had half as many roof bows as the school buses because of the transit style slider windows they had. I would guess with a 300 HP engine and half the framing that bus was probably pretty limber after more than 20-years of service.