new rule
annoyed that the mother didn't call you before you left, absolutely.
upset - more sad for my daughter that she had one more disruption in her life.
mad - definitely not.
It was that little girl's first day too. She probably wanted the excitement of riding the bus.
Did you confirm the plans with the mother the day before? I am in a carpool and last minute changes are the norm. The first day of school is always different than the regular carpool due to the excitement of it.
I agree that this is a minor thing and definitely not something your daughter should have been crying so hard about. Did she not see her friend a few minutes later when she got to school?
Edited to add: although I can completely understand now that I read the OP's answer how a disappointment like this could be the proverbial straw that broke the camel's back.
Have you considered some counseling for your daughter to help her deal with all the stresses she is dealing with? That is quite a bit to deal with at 7. The school social worker might be able to give you some tips.