Do you want to pay more in taxes duebto the overtime you would have to pay the maintenance workers to come in earlier? It is all cost benefit. Add in the liability issues if something goes wrong or a kid doesn't get picked up or whatever. Sucks to make that call but sometimes you just have to make it.
It is NOT all tied to finances, but rather safety for the students. I drove a school bus, so I can speak from some limited experience. Sometimes, even if the buses do start, the fuel sitting in the tank is still in a somewhat gelled state and the buses will not continue to run if fuel reaches the engine in that state. There are additives that are used to stop that, but it doesn't always work. It depends on where the fuel was refined and processed, and the amount of additive they put in that fuel during processing for the winter months. Even if fuel is purchased from the same source all the time, (and it usually is), the mixtures may be different from batch to batch. For example, a couple of weeks ago, one or two school districts had no problems with their buses starting and running, while several other districts did.
The extra time allows for experimentation with additive combinations to see if they will work over an extended period of time. Just because a bus will start, doesn't mean it will continue to run during an entire route. The schools, (and parents), don't want to take a chance on students being stranded on a frigid bus waiting for an alternative ride.