This has been my experience as well. It's either retirees or stay at home moms/dads that drive the buses part time. They may not be purely volunteer (no pay), but it's definitely a part time job. Couple of hours in the morning and afternoon.

Also, it seems to be mostly in rural areas. Some drivers keep the buses at home during the school year. And, these are large, full size buses. If you live in a medium sized or large city, it's probably different.