I don't know what this woman was supposed to do. The field trip ran late, which presumably made her late for picking up her own kids, for which there can be penalties, or safety issues. So she has the choice to either stop and get them on the way, which it sounds like she did, or drive the school kids back, drop them off, then go back for her kids, making her possibly 20 minutes later in the other direction to get her own kids. Depending on how old they are could be stuck in the principal's office with some overzealous teacher calling the authorities because their parent hasn't picked them up (no cell phone coverage, so the mum couldn't be called herself to find out where she was) or, if they're older, may have been waiting outside the school gates on their own. I think she did the best she could in a difficult situation.
I agree with those who say if the OP wants that much control over a few minutes, she should drive the kids herself.