[DanaC]

Another lesson that your kid mayhave learned (and which you can help him learn) is that by telling the truth and admitting his part in the incident he was only given a very mild punishment. His honesty bought a good deal more leniency than had he lied and still been found out.

As unfair as it is (and it's the kind of unfairness that gets under your skin and itches terribly) that kidB wasn't punished, it almost doesn't matter: your kid (kidA) allowed himself, for whatever reason, to be drawn into a situation where he did hit kid C. That was wrong; he clearly recognises that it was wrong; he has gone out of his way to make amends.

In rationalising it to him, it might be worth pointing out that whilst he (an honest and mostly well-behaved kid) has ended up being punished and Kid B hasn't: in fact the kid who has been treated most unfairly is KidB. By not punishing him, and allowing him to get away with that behaviour; the school and headteacher have taught him a dangerous lesson. By the time he comes a cropper properly, it will most likely be for something serious, and the consequences will hit him like a freight train.

Given that part of the school's job is to socialise kids and allow them to move comfortably through society, the response of the school to misbehaviour is just one more set of lessons. Your kid was given that lesson. Kid B was not. Better to learn those lessons now whilst the potential consequences are small, than continue through school without learning those lessons and then have to learn them in a much bigger and potentially damaging way later on.

As a final aside: it also could feed into the lesson that actually, what other people do is way less important than what you yourself do. Why would you care about the other kid, after all? What matters to you is what your kid is doing; what lessons he is learning. In much the same way as parents don't care that kids E, F, G and H all have brand new i-phones to take to school, or that kids E, F and H told kid A to do something: if kids E,F and H told ya to jump off a cliff would ya?

The only thing your kid can control is his own behaviour and responses. Ultimately responsibility for our own actions cannot rest with the crowd, it has rest with ourselves.