My first response would be "Nothing important" since the air line is cut (and presumably the remainder is missing). A more proper answer is it's probably an air-relay, likely for the air-actuated stop sign or crossing bar which may have been removed or disabled by the school district prior to decommissioning the bus. I wouldn't be surprised if the wire going to it was "dead" too, either cut somewhere or had a fuse pulled.
definitely like the last post an air relay... on my bus i had one like it under the hood for the srtop sign... also if its near the firewall many busses came with "air shutters" that closed during cold weather... these became problemartic as they aged.. would stick and rust so people would take them off... they used an air-relay also..
well the signs worked when I bought the bus, I have since removed the power to that switch and took it out. The 8 way system for the lights is gone too. So if it is in fact a relay for the stop signs, it shouldn't hurt anything if i remove it completely, right? I had the motor replaced and a new ecm programmed for it. I plan on taking the signs off next weekend.
I took the air relays off for my stop signs.. I traced the air lines going back to the cab of the bus and knew that I had no reason for air to be in the cab.. I have no air seat and my air ride suspension I had already found where it goes.. I started pulling o nthe line and it popped out as an empty ended hose.. so I yanked the air relay and tagged the wires so in case i need a wire for somethign under the hood I knew the other ends went to the stop sign and school-lights contreoller I bad removed... I saved the valve relay too as I might at some point need one for something..
It's a control solenoid for the cooling fan. By removing the single wire you open the solenoid allowing air pressure to go to the fan clutch engaging the cooling fan at full speed. When the wire is hooked up the ecm controls the fan. If I'm correct there should be an air line that goes to the fan clutch. The cut off piece of air hose is just an exhaust tube