In my past integrator life, we won a school bus bid. There was a tech that had recently come on who used to work for the city comm center installing radios in police cars. So he could get buses done fairly quickly and there wasn't a ton of profit, but it was healthy. I think it's gotten more competitive since then. This was in the late 2000s.
I've heard of another integrator who hired a car stereo installer to do all the rough work, since those guys are more suited than typical techs. Plus they generally are low paid, so anything more lucrative they jumped at. The integrator did the config in house, installer ran cables and put up cameras, and they came out and did a final check on each bus.
I do think it's a shame that so many of these go to the lowest bidder, though. They're big projects that end up being standalone instead of part of a school or city's overall security plan.