Your kid throwing rocks. Imagine. That particular case had nothing to do with school. However, as I stated, if an altercation at a school leads to a fight after school even off of school property the school can discipline the children.
Ask yourself what happens to the athlete who gets caught drinking on the weekend. The school has authority to bench that kid for the act according to a code of conduct.
Again, rather you agree with the term of punishment you will learn in the longrun the school DID have the RIGHT to act in this case.