S.E.
I am a former school teacher and in my experience, my school was very good about taking responsibility when it was ours. I cannot speak for other schools but I can say that public schools have very specific rules and procedures for just about any incident. There isn't much wiggle room!
So, having said that, I want to first say that of course you're upset and it's an upsetting situation. Second, as I'm sure you know, you catch more flies with honey. Try to exercise patience and understanding. Thankfully your nephew is physically okay (albeit a little banged up) so this is not an emergency situation. The more calm you are, the less defensive the school system will be.
You may consider speaking with the administrative offices of your school system to find out who to speak with and even get several names, titles, and phone numbers. Document everything!
Good luck!