Do you think the Head is unaware of the situation? If he is, then he is the worst Head ever and any letter you send will not help one Iota. If he is aware of the situation (the more likely scenario), then how do you think he will react to your letter? Bear in mind you have only been in the school a week. The school obviously has discipline issues and they need to be addressed. Do you have any solutions in mind? or would you simply be writing a letter pointing out what everybody at the school probably already knows?