I don't know if that's regional or even totally different in every school district, but I've lived in 3 different school districts, and had never heard of high school and elementary kids sharing a bus. No way would I let DD ride if that were the case!
I thought about how it would work here, logistically. We have only one elementary school, one middle school, and one high school, but they are nowhere near each other. If one bus had to go to all three schools to pick up kids, it would be nearly an hour after school let out before the bus even STARTED taking the kids home. It would mean some kids would be on the bus for 2 hours, and the kids at the school where pick-up was last would have to wait 45 minutes after school was out to get on the bus. Who would supervise them all that time? It never occurred to me that in some towns the schools would be close enough together for one bus to pick up from several schools. Besides, our elementary school alone fills 6 buses. I can't imagine squishing older kids on there too.
Also, our bus drops off in front of each house; there are no bus stops. So DD is dropped off at our front door. We did have had a one time incident of bullying when she was in 2nd grade, but after I notified the school it never happened again. Our principal doesn't put up with that, and our bus driver is good, too. If that weren't the case, I would have second thoughts about the bus.
It's really interesting to hear the ways school is "done" in different places. Most interesting to me is the different start and ending times. I'm 42, and when I was a child,. school was 8-3, and now for my kids, it's 8-3. I just assumed that was standard everywhere. I had never heard of anything different except here on the DIS.