The logical fallacy here is the assumption that teachers will always have pupils’ best interests at heart. There’s little to stop children from becoming extremely vulnerable if they are under the supervision of someone who could turn on them. Gun attacks like Columbine and Virginia Tech are often by people whose potential for violence was not spotted by anyone until it was too late. People in positions of authority are not always reliable or rational, and no amount of safety checks can guarantee that some teachers will not abuse the powers they have. This measure would simply increase the potential threat from those who have been authorised to carry guns in schools.