Registration will be no problem, you can register it as a private bus, van, or suburban type vehicle. My bus was registered as a suburban when I bought it.
As for insurance you will just register it as a truck not a school bus. Give them the year, make and model of the chassis and then you are usually good. A lot of farmers use school buses for hauling things and carrying people around most states even have special laws allowing for this.
It sounds to me that school bus will be an excellent vehicle for your organizaion. They can carry a LOT of weight, cheap to buy and mantain, been maintained to govt standards, and best of all you can easily customize it to suit exactly what you want