In the world of commercial vehicles, school buses are number 2 behind garbage trucks as the most severe service.
In most calculations the mileage of a school bus is equal to 2x to 3x of what an over the road truck mileage and abuse would be.
The fact the vast majority of school buses have numerous starts and stops every day, more in a day than most over the road trucks have in a week.
I once drove a route that had seven different parts to it with a grand total of 27 miles per day.
I would start the bus and warm it up at the bus garage, go out and make three stops and go to the high school/middle school where I would shut the bus off. Thirty minutes later I would restart the bus and make seven stops on the way to the elementary school where I would shut the bus off again. 45-minutes later I would do another special kindergarten run that had nine stops. After dropping off at a different elementary school I went back to the bus garage. 90-minutes later I started up and did a mid-day kindergarten run that started at the school and picked up and dropped off 14 stops and then back the bus garage where I had an hour layover. I then restarted to do a shuttle between middle schools after which I shut down for an hour before the afternoon high school run. After the high school run I had a 30-minute layover at the elementary school where I finished up the day after seven more stops.
Grand total for the day was 27 miles.
In the winter it never ever got very warm in the bus because it never ran long enough or hard enough to get hot.
And by definition that was pretty hard miles. It was balanced by the fact I also did a lot of extra trips and they usually dispatched me in my bus for those trips. Which meant it usually got at least one trip per week that was at least 50 miles in one direction.