While I see where you are going with this, a couple things about this stat; One, there are, most likely, less airplanes flying around at any one time than there are classrooms full of children in America. Second, a plane is, by definition, a higher risk environment than a classroom (unless that classroom happens to be a mechanically powered one that is currently situated 30,000 feet above terra firma and not resting on any type of landmass) and also easily converted into an actual weapon (see: 9/11/01). Thusly, it is both logical and understandable that a plane would be a more well guarded environment than a classroom.