I just bought this. Allows you to trace wires. It's a Fluke, good brand, and I'm really impressed with how well it's built and works.
I got it at Lowe's for $72, Amazon for $89.
The thing about a school bus is the "safety" things for delivering kids (stop sign, crossing bar, flashing reds, etc.) are not needed to "safely" run the actual bus.
Yet, the safety systems are so integrated in with the bus that it's a real project to remove the safety system.
There is a thread for re-wiring the high up flashing alternating red and yellow lights so they are steady on/off.
Things like the crossing guard and stop sign could be traced back to the panel and I think safely disconnected. Each time you take out a wire, make sure the bus still starts and works!
Personally, I like the emergency door interlock system because the bus won't start if the doors sliding locks are engaged. Keeps any potential thieves when the buzzers start blaring and the engine won't turn.
Hope this helps.
Best of luck.