No one should be allowed to give a "lawfull" oder to bar them from taking their kids home. In all reality, schools are a target, and the dirtbag will come in and try to take kids hostage whether the guy at the door tells him to or not... remember he has a gun, the guy at the door(who would usually be a school admin) probly doesnt.
In fact the guy at the door (unless he is an LEO) is barred by law from having a gun.