The best thing you can do is what you are doing: go to the headmaster, relate the story as you know it, explain your concerns. Bear in mind that, for the confidentiality reasons you've already cited, only the teaching staff in the school can see the big picture here. Bear in mind, too, that if the other kid gets sent to a special school, the road back from there to normal society is a very rarely-travelled one: expulsion, especially from primary schools, is usually used only in provably irredeemable cases.

I wouldn't mention the "closing ranks" thing, or the thing about the kid's father being a governor. I wouldn't demand "real action". Comments like that will detract from the valid concerns that you do have.

I don't think you'll have any luck taking your story to even the feeblest local newspaper, for the sad-but-true reason that it's not news: everything you've described happens in every school in the country on a regular basis. Again, unless the incidents you describe are part of a wider pattern, then I think it's certain that the other kid will be disciplined, possibly even suspended, but unlikely that he'd be expelled.

When I was at primary school there was a kid in my class who was a bit like your DP sounds. He lived his life with the random violence which he'd experienced in his own home life. He beat everyone up at least once, including me. He didn't get expelled (I guess he might have had suspensions, but I don't remember it). Then suddenly, at age 11, to everyone's surprise including his own, he discovered he had an amazing singing voice. He sang beautiful solos in several school productions and the violence just stopped. Stopped. I'm sure that if they'd bundled him off to a special school at age 10 he'd be in prison today.

I'm not trying to claim that the problems here are minor, but they may be less serious than you think. (From your comment about "not raising the profile" of it with your own child, it sounds like he wasn't too traumatised.) If I were you, I'd express my concerns and then trust the teachers. Anger management is their job.

Peter