Busses should not be driven on sand, too much weight on too narrow of tires. It would be like trying to drive through a couple feet of snow. They however to ok on grassland, even mushey wet land as long as you keep your speed faster than one might like, and eventually you will get very stuck!
When taking a vehicle like a bus off road, especially without a locker in the rear, you WILL get stuck eventually and you will realize just how heavy a bus is.
On gravel and dirt roads school busses do ok. The ride is very rough, but 60mph feels perfectly safe.
I would also recommend a manual transmision.