Since when is a 17 year old a child? If the state brings rape charges against him are you gonna still call him a child?
I don't think the father acted responsibly. What he did in front of other minors at school was wrong. He was lucky he only busted this kid's face up a bit. What if his blow had caused an aneurysm? What if he knocked the kid to the pavement and caused brain damage? Would it still be sensible then? Would the courts justify manslaughter because a 17 year old slept with a 15 year old?
Prosecute the kid for statutory rape under the law. Prosecute the dad for battery under the law. Let both of them take responsibility for their actions.