Anonymous wrote:OP, these three did not rearrange their schedules in order to avoid sharing a shift with you. More likely they saw someone else had volunteered and switched to a slot that didn’t have coverage. It’s possible they wanted to volunteer together for some reason, and that’s why they all changed (there are certain times at our book fair that are basically dead, so they can be a good chance to have a quick PTA work session while also providing volunteer coverage).
They all switched to the time slot right after - all three of them. Leaving OP as the only volunteer in her time slot. Unless they ended up with a meeting during the earlier time slot, it's at the very least kind of strange.
OP, I'd show up and see what happens. If it's a PTA filled with a bunch of jerks, better to find out right off the bat.