A "schoolie" is what they are called, and they are fairly common.
A school bus is relatively inexpensive to buy used, and the chassis and body are well made and durable. The powertrain is rubust, and the fleets are normally very well maintained, so a used bus will typically have a lot of miles left in it.
Plumbing can be ran under kitchen cabinets, sometimes a false floor is studded in the bathroom to run plumbing under.
Sheets of foam insulation can help keep it comfortable, and standard rv components like air conditioners, furnaces, fridges and water heaters can be used.