You don't give a lot of info here. How many in your family? Will you be camping at a campground or boondocking? Since you're taking the school bus to school, do you even have a driver's license?
Obviously, you want a good, reliable engine with not too many miles on the clock. Diesels typically last three times what a gas engine does so 200,000 miles is actually not that much. The bolt on's (alternator, water pump, transmission, etc.) aren't usually built to match so you may get an engine with lots of life left in it but the other stuff needs work. Look for tires and brakes with some life left in them because replaceing them is way more expensive than on a car. Assume that you'll have to do a fair bit of maintenance right off the bat. Fleet maintained vehicles are sought after but the maintenance crews know which bus is coming out of service and with service budgets shrinking the scheduled maintenance often doesn't get done. Plan on replacing all filters and fluids at the very least.
In many states you're required to paint over the school bus yellow so keep that in mind. Your intended use will dictate which bus you need.