It would certianly solve the problem with outsiders coming into the school to do violence however I'd be afraid of the urge by many of us teachers to turn it on some of our students.
You're a teacher? I hope you're being sarcastic. I carry a gun when I'm not at work as a teacher in an inner city senior high. When I carry I am extremely aware of my responsibilities to society to safely handle the firearm and use it only in the most dire of life threatening situations. Consequently, I am extra careful to neither give nor take offense at anything. I'm ultra courteous.
When I work, I'm sometimes irritated by the kids, but never truly angry. It would never occur to me to become physically violent with a student except in self defense. I have done this a number of times. Aikido is a wonderful martial art! In the classroom I'm extremely aware of my responsibilities and duties. Any teacher who is at risk of harming a student in an offensive manner which is to say by initiating violence shouldn't be teaching.