I have a rule that if they arrive within the 15 mins before we leave for the school run, they have to dress their child themselves in the winter gear. My own kids have been late for school many times due to unprepared parents. In the winter it takes longer for the school walk, so I have them in my garage and strapped into the stroller earlier with the front door locked, so if a parent arrives, they have to get ready fast, or we leave. No discussion. Once they get left behind once, they learn their lesson. The parent must dress the child, and be ready to go. You can't have the kids miss the bus!