You are talking about school rules that you are drawing up for your school. I think it is better to say, "Our boys are expected to keep their hair short and tidy. Long hair is not allowed in this school." This way of putting it is positive and honest. It also minimises any damage that may cause to their mental development. Regarding the "damage" part, let me explain: 1. There is nothing intrinsically good or bad about long hair for boys, or boys or men with long hair. As a responsible teacher you certainly should not inculcate in them the bigoted and twisted view that "boys should not have long hair". By limiting the rule to being merely one school's preference and one school's rule, you are letting them know that it is not a universal imperative and that they may personally be open-minded about it. 2. By so doing, you teach them that while they are members of an organisation or a team, they should abide by the rules, as a matter of pragmatic survival, but it does not mean that they have to believe in the rules or change their own view of the world. You are shaping young lives. It is a highly important and delicate task.

You can word it that way. When you add the comma after the word , boys. It seperates them from the context of the sentence. For example ... "Boys, don't keep long hair. " Indicates that you're talking to boys and telling them not to keep their hair long. "Boys don't keep their hair long. " This is how you would say it if you wern't talking directly to the subject, as if you are saying that boys in general just don't keep their hair long.