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.
There's an even easier solution to that: IIRC elementary school kids are way more resistant to sleep deprivation than teenagers, so you can just flip the schedules around and start elementary school kids before teenagers. Downside is probably then that the more-demanding elementary school kids get home earlier, but I think they're already getting home well before the working parent does.
Our school system does this. I haven't heard any complaints about it. (And we have a lot of entitled parents who complain about everything else.) I'm surprised that more schools aren't switching.
I've heard that the reason this isn't the default is that parents get far more upset about making an elementary school kid get up and wait for a bus in the dark than a teenager.
I understand the arguments are made about the danger of standing next to a road invisibly (especially in shitty places with no sidewalks) but I'm sure some people are worried about kidnapping!
Heard that argument here when the idea was discussed. The funny thing is that these days (at least for Elementary school) there's always a parent or two who walks their kid to the bus stop and waits with them until the bus comes.
