I'm not an expert on Blue Bird wiring - I just used to put radios in them. I would go back to your BB mechanic and ask about the next steps. Tell him everything you have tried, and ask for more suggestions.
If it was only no start, I understand lots of buses will not start if the rear door is not properly set to transport children, There is a lot of heartburn expressed here on Skoolies trying to find the way properly bypass the starter lock-out wiring when disconnecting the buzzers.
But if the whole dash is completely dead, it seems like maybe a hot wire at the back door touched the chassis, and blew a fuse or a fusible link. Fusible links are pieces of wire designed to burn out as a last resort if fuses don't protect the wiring, and they give up the ghost instead of having a vehicle go up in flames. I don't know if BB uses them or not. Your expert should know.
Worst case, if you have a dash that uses printed circuit boards, and had shorted a wire that passes through the PCB on the way to the exit door, one of the copper strips could have burned up. These can be located by careful visual inspection once they are out of the dash. Burned circuit boards can usually be repaired by removing the damaged section, scraping the varnish off of the good copper to the right and left of the burned section, and soldering a piece of wire across the missing part.
In hindsight, testing the starter with a screwdriver probably should have been tried before replacing the old one.