I mean, it is a valid reason. My small town (4k~ people) did the same, high school started at 7:45am and each bus had both a high school / middle school route, and immediately after an elementary school route. Not only would you need twice as many busses, you'd need twice as many drivers. The cost difference is massive. That said, that doesn't mean they can't bump the start time of all the schools up an hour or two.