I'm just surprised they're allowed to have people in the cabs at all, even with supervision.

Imagine explaining to the insurance what happened if someone was to let the handbrake off and it ran into another bus, or the courts if someone let off the handbrake and it ran over a bunch of school children.
I'm pretty sure the person 'supervising' would not be let off lightly in such an incident since I believe legally they'd be seen as in control of the vehicle, the same way that a driving instructor is when teaching a student.