Hey, wait a second. Was he using the pen to write a threatening letter? If so, then I understand. Gun's aren't supposed to be used as a threat, so neither should the pen with the firearm logo.
BUT...if he was doing his schoolwork, then who gives a flying fornication about what kind of pen was in his hand? I know of some kids in my high school who were caught with their little soldier standing at attention in their hands, and were only suspended. I know of a couple that engaged in sexual activities in the class room, and the teach turned a blind eye to one, the other got suspended. SUSPENDED, not expelled. Everyone in my high school fought, causing physical harm to one another, and no one was expelled. People threw around racial slurs at my high school, and threatened a race riot, and I think maybe one of the staff scratched their ass on that. Why the hell are we punishing the youth of today for such idiotic and frankly ignorant reasons? All we are doing is laying the breeding grounds for a rebellion against all authority in the future.