Before starter was pulled (and taken out of the loop)
It should have been jumped across as SomewhereinUSA stated...
If you're not sure how to jump a starter solenoid, Google is your friend.
The scenario SomewhereinUSA was talking about is:
1) turn ignition to RUN
2) jump starter solenoid. If it starts & runs, it tells you that connection is good. If it does not start & run, then it's a switch (or bad wire) breaking the connection.
But, keep in mind...
You're trying to start something that hasn't run in years... You can only guess what caused it's removal from service... What was scavenged off of it before it left the school yard, etc.
Did a mechanic pull a part he needed for another bus, to keep it going?
Lot of variables!