You should have a gun in every classroom, concealed on the teacher, for the same reason you have fire extinguishers and first aid kits.  You probably don't think about them often and have no plan to use any of these items, but if the occasion presents itself that you need to use any one of these, you will be thankful that it is available.

A weapon on the teacher is sort of like air, you never think about it until it is not there and you need it.  Having a weapon nearby is part of being prepared.

Consider item 5 of the list of things needed for REAL PROTECTION for our school children.  "Protection is proportional to participation."  This means that the more teachers we can arm in a school, the safer the school will be.  The idea is that we need to be able to overwhelm the killer when he enters the school.  The best protection is when the killer is resisted by so many armed people that he quits, dies quickly, or just decides to go elsewhere to kill people.  An overwhelming response to the killer is also the best way to minimize casualties among the school staff and assure that medical aid arrives quickly.

As a lesson in reality, consider the situation at Sandy Hook Elementary.  The deranged killer shot out the front window to get into the school.  The noise he made caused Principal Dawn Hochsprung, school psychologist Mary Sherlach, and lead teacher Natalie Hammond to exit the conference room, where they were meeting with other faculty members. They went out into the hall to determine the source of the sounds, which they did not recognize as gunshots.  It was a three to one battle which should have been easily won by the three women, except that liberal state legislators had passed laws totally disarming them (to keep them safe).   The evil murderer killed Principal Hochsprung and psychologist Sherlach on the spot.  He then shot the lead teacher Hammond twice (she survived by playing dead and then crawling back into the conference room while the murderer continued to kill other people).

If these three brave women had been armed, they likely would have come into the hallway more cautiously because as part of their weapon's training they would have learned what gunshots sound like.  The battle would have been more than three to one on their side, since others in the conference room would also have been armed, and these ladies could have overwhelmed the killer and prevented any student deaths. 

So, why have a gun on every teacher and staff member?  Because it makes the children very safe and puts the fear of God in any would-be, deranged killer.  Going into a school to play out your video game fantasy of shooting innocent children is not so appealing when the adults in that school can quickly put a big hurt on you.  That is called deterrence, and it works.