You can see the head's problem - divided loyalties. Heads have a responsibility to the children in their care and are obliged to act in their best interests, but they are also the senior manager of a group of professionals and must act to protect their welfare. The unhappy incident you describe brings these two loyalties into head-on collision.
It sounds as if the boy has been in trouble before and is on a "yellow card". Keeping the matter quiet is not always the best solution to repeated misdemeanours as procrastination can cause three sets of problems: aggrieved teachers and no doubt their very angry union representatives; fellow pupils fearful for their own safety if it appears that someone can safely swing a punch at a teacher and get away with it; and an aggressive boy, who may well believe that anything goes, since no one is prepared to define some limits.
You should insist that the matter is dealt with properly. If the events were exactly as you describe, then you deserve all the protection that the school's own rules, and indeed the law itself, can offer. The head may be fearful that the governors will take a strong line and permanently exclude the pupil, thus giving him a stigma early in his life which may jeopardise his career. But the boy is entitled to a fair hearing. He may have a medical problem, or need special help managing his inner violence. Simply ignoring his violent behaviour does him no good either.
If you feel diffident about raising the issue, seek help from your union, or from an experienced and understanding colleague. The school, including head and governors, must face up to its responsibilities to the whole community. These are difficult and stressful matters, but the ostrich response is no solution.
Inform your union
You should not leave. Why allow the pugilist another direct hit? What you should do is make your union aware of the incident (you are a member of a union, aren't you?) and let them handle the issue on your behalf.
Ralph Moses, Manchester
Seek staff support
If you turn yourself into a martyr by resigning, you may not actually achieve very much for the school, and you may jeopardise your future job prospects. Assuming that this particular pupil has a history of indiscipline, any action should involve the entire staff.
Insist on discussing the matter at a regular or extraordinary staff meeting and make your feelings clear. In matters of the enforcement of discipline, teachers should act in concert and, in extreme cases, refuse to teach recalcitrant pupils. Managers cannot discipline the entire school staff, who have the power to close the school if this is deemed necessary.
Anthony Ireland, Lancashire
It is the head's decision
I have to agree with the head. I've worked for several years in both care and education sectors and have been in several similar situations. If you feel that you need to physically intervene in a situation, then you have to accept that you may be hurt.
But it's a different scenario if you managed to break up the argument and were then punched. I'd then disagree with the head, who has a duty to protect his staff. The boy would need to be sanctioned appropriately and, if you're not personally satisfied with the head's decision, feel free to report the incident to the police, who may prosecute on your behalf.