One issue that occurs to me is... what if the OP, or the parents of other kids in the late car, had also needed to pick up kids at a certain time? I think that mostly this was the fault of the preschool not ending their field trip on time. Getting back late, when parents have made plans for pickups, etc., can really put parents in a tight spot. The mom did try to call, after all, and wouldn't have known that the director didn't receive the message until later. And as for leaving a message instead of talking to a person... if the only number she had wasn't being picked up, exactly how was she going to force the director to answer? I don't really blame her as much as the other people on here. She's going to have to worry *somebody*, either her kids, her kids' school, or the carpooled kids' parents. And all because the field trip went later than it was scheduled to.
I do think the best thing might have been to tell somebody as they were leaving the field trip location. They might have either given her permission, helped her to shuffle her carpooled kids into somebody else's car, or called her older kids' school to explain the situation, teacher-to-teacher. But I could certainly see, say, the field trip ending at 1:30, but then the mom has to take the kids to the bathroom, and walk a long way to/through the parking lot, one kid is whining and tired and walking at a snail's pace, then you get the kids buckled into their carseats, and now it's 2:10 and all of a sudden you have a 50-minute drive back, but you're also supposed to pick up your kids at 3pm sharp, etc.