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.