I am a school bus driver, so I feel I can weigh in on this. The short buses we have all have a driver door that locks. Nearly all of them have a service door (the door the children enter through) that opens via an arm the driver slides out. On most of them, this arm "locks" into place when fully closed. However, I've yet to see any bus with a locking rear door. In fact, I've driven a big bus before that had such an arm (but no driver door), and someone closed the door all the way and I had to enter the bus through the rear door to open the service door. Some of the short buses have a door that's opened via an electrical switch, like most of the big buses. When the bus is turned off, it may or may not be possible to close the door without the emergency switch being activated (it allows the door to be pushed/pulled open manually). Most drivers of big buses just flip this switch as they're exiting the bus, then push the doors closed behind them, but anyone can come along and reopen them. We do have anti-theft systems installed on all of our buses, though. If the motion sensors detect motion, an audible voice says, "Leave vehicle now! Authorities notified!" and if they don't leave, the horn starts going off.
You would likely have to modify the bus, unless you have a commercial driver license. Vehicles designed to carry 15 or more passengers and the driver typically require a CDL with passenger endorsement. I've never seen one of the small buses with air brakes, but that's something to look into, though I'd imagine most of them are small enough not to need them.
My company has three locations, at one of our locations, we have small buses that use gasoline. At the other two, they're all diesel. That's also something to consider, as diesel fuel tends to be more expensive than gasoline.