It may be a difficult thing for parents to accept, but it's common for kids (especially as they approach the teen years) to use bad language when they're with their friends as a way of demonstrating how "cool" and independent they are. You need to teach that it is inappropriate to curse, especially in school, in church, or around adults. While we understand your instinct to not make too a big a deal out of this relatively small infraction, you should still talk with your son about it. Explain that you know he probably swears when he's with his friends, but that bad language does not belong in school under any circumstances. Since you say the offensive word was written in a note to friends and not directed at the teacher, then you're probably right not to overreact. Just make it clear to your son that if he uses that type of language in school again for any reason, he most certainly will be punished.