Looks like a 50-60 passenger bus, so I would guess it has a GVWR of about 26,000 lbs. The engine is some sort of Navistar Diesel V8. It is probably an automatic transmission, as most school buses have long since abandoned manuals, which probably means some sort of Allison, probably a 3-speed. The rear axle probably has a gear ratio of around 4:1, as most school buses don't need to run at more than 50mph (are generally restricted by law in how fast they can go with students on board, so the companies don't bother to set them up to go faster). You can get it up to 60, but it will be screaming.
It most likely has around 60,000-80,000 miles, again because schools can only run buses for 5 years before they must replace them, and most bus routes are in the 20-mile or less range, and whoever got it after will very likely not have driven it much.