School Bus Mysteries Solved

The experience of chaperoning my third-grader's school field trip last Friday solved two mysteries for me. When I mentioned this epiphany to my wife, she said "well, duh", and maybe you will, too. But maybe you're like me and had always wondered:

1. Why do school field trips leave the school at, like 9:30 and come back to the school at, say, 2:00, when the school day runs 9 am to 3:40?
2. Just what do school bus drivers do between the morning and afternoon runs?

It turns out the two mysteries and their solutions intertwine. Maybe this only applies in districts like ours that contract out every single bus trip; the district does not own buses. You've picked up on it, right? The buses can only be rented out for field trip runs at times that don't conflict with the morning and afternoon runs. So, to be safe, the bus company won't guarantee they can leave my son's school until 9:30, after their drivers have dropped off their morning kids and driven to the school from wherever. The buses that drove for the field trip are different and come from different companies than the buses that drive my son and his classmates to school.

Of course, solving one or two mysteries sometimes opens up many more. I'm still wondering:

a. where the buses/drivers go during the field trip time? other shorter field trips nested within the field trips?
b. what crossing guards do in the middle of the day?
c. why the field trips from my kid's school always leave the school 30 minutes later than they say they will? I walked into the museum on Friday and saw two parents waiting there. I heard one tell the other that she'd been there since 9:30, the time the kids were supposed to leave the school. I asked "first time chaperoning a field trip?" They sighed that it was. I shared my wisdom about not rushing to field trip locations and then proceeded to wait another 30 minutes with them until the buses rolled up.