So report it to the rank above your dept head. Document everything, the bending of the rules, the friendship and the resulting harassment you are now being subjected to. If you have to, take it to the body that oversees your school district. Hell, leak it to the local media and time how fast the negative publicity gets it fixed...

Are you suggesting that a known bad actor might retaliate against someone who reports them - As the reporting system is in place to facilitate? *Shock of shocks*. If only there were perhaps ways to file follow-up reports about said retaliation. If the system continues to suck and protect the department head who is fine with kids being attacked, so long as it's by kids of friends of his, then, yeah, that's probably not a system to stay in. But, like, I'm guessing the alternative of "becoming buddies with a department head who lets violence against kids slide" probably isn't an appealing option to OP, either.