In the US, you can drive a vehicle with up to 15 passengers on a regular license.

The bus has been converted and should be registered as a non-commercial recreational vehicle, so it would be the same license for that... probably a Class A, depending on gross vehicle weight rating.

It's a slight step up in training from your normal vehicle, and would probably require a health certificate, etc... but not as expensive as a CDL (commercial driver license), which would usually entail a few weeks of training and ~$2,500 in costs.