You could try seeing if they have a wiring diagram online, http://service.blue-bird.com/wiringschematics/
(you'll need the body number, which is on that ID tag on the compartment over the windshield, just above the entrance door). In a 94, usually the lights would have a positive wire going from the fuse/breaker to the switch, then to each light. The lights are then grounded to the body. If the wiring's that simple, it could be that the switch is bad (try disconnecting the wires from the back of the switch).
However, you mentioned the rear door - if it's normal for the lights to come on when that's open, there will be extra wiring. I'm not that familiar with Blue Bird wiring, but there may be a separate relay that's grounded by the door switch, or the lights could be wired with ground-side switching (positive wire directly to the lights, and a ground wire to each switch). With the key on, does the emergency door buzzer stay on? That could mean either a bad door switch or a short somewhere along the body wiring.
Also, I'm not sure if you meant you checked the chassis or body fuses, but all the wiring for the dome lights would be behind the left-side switch panel. Chassis wiring is in the dash near the gauges, body wiring (bus warning lights, dome lights, clearance lights, connections for stop/turn/tail/backup lights, and all heaters, defrosters, and fans) is behind that switch panel.