J.S.
What does your daughter say? Is she really being mean or is the other girl hurt because your daughter doesn't pay that much attention to her at school?
Something is weird with this story. The girls are fine in the car, but the other kid turns around and tells mom she's being bullied by your daughter at school.
Maybe the carpooling should stop.
ETA: and if it's just hurt feelings, tell the other mom that your daughter is not bullying hers. She just has different friends at school and that's okay.