My understanding of the situation:
1. It was only "school sanctioned" inasmuch as they let school out so that kids could attend.
2. The kid did not attend school the day this occurred, so even if this was a "school sanctioned" event, he was not part of it.
3. He was standing on public property while he did this, not any property belonging to the school.
That being said, I hope the school pays!