It is vital that the engine cooling system is full of coolant -- water & anti-freeze -- with no significant pockets of air.
As mentioned already, you can run the engine and top off the water until you are certain.
Most thermostats have a small bypass hole, but I like to drill it out a bit. And two of them, top and bottom, if the thermostat is mounted vertically, as is the case on Millicent's Cummins 5.9.
Some buses have bleeder valves at the high points of heater hoses, where air becomes trapped when the hose is filled. Such bleeder valves can certainly be added.
If you still have heater hoses running over the left front wheel-well in a flat-nose bus, that's a high spot right there.
The in-dash heater may be a big one, depending on the bus.
Be sure to open all heater valves while "burping" the system.
Some years ago, I had replaced Millicent's water pump, and we were in a hurry to leave on a trip. Yup.... After about a mile, she overheated rather seriously.
That was when I added holes to the thermostat, to speed "burping".