Do you get a click in the starter solonoid?
It’s the small barrel attached to the starter.
If that’s on the way out when you turn the key in the ignition you’ll get a fairly loud click in the back of the bus but the starter won’t turn over. After trying a few times with the key eventually and the solonoid will kick over the starter and engine will turn over/start.
You can start it manually by shorting the solenoid contacts, see idiots guide book.
Just another option to check.