No, we don't. You made the argument yourself, that a school is legally responsible for the welfare of students while in its care. Is a student on his parents' property outside of school hours in the school's care? Obviously not. Is a student on a public street who is waiting for a school bus, without school supervision (there was no school monitor or guard there) under the school's care? Obviously not.
I don't think we know the law well enough to say that. I'm going to look this up tonight. Laws in relation to schools can be weird.
In memory of Ahmed Merabet, Rafael Ramos, and Wenjian Liu.