Can you help with this carpool etiquette question?

We carpool with one other family in the neighborhood, for the last two school years they have taken the kids to school and we have picked up. I have one son in middle school with her boy, and another who goes to the high school nearby. The starting and ending times are perfectly staggered so they can all be dropped off and picked up in one trip. We also watch their elementary school daughter in the morning before school starts. With about a month left of school, our carpool partner took a new job with hours that don’t let her pick up or drop off, so as a result I am driving all of our children both ways.

This evening my son informed me that their child has a meeting after school and needs to stay an extra hour. He would like me to come back and pick him up at that time. My son in middle school would stay the extra hour, but I would still need to pick up the high schooler. I know for sure the father works hours that make it possible for him to get his boy after the meeting.

I am inclined to say no to having my son wait and making the extra trip. But I am not sure if I am bound by common courtesy or carpool rules to say yes.