That's weird, I've been told only a man with a gun stops a shooter?
It is true that an armed teacher or other staff who are inside the school and in the line of fire are more likely to use a gun without needing any pep talk.
Even being dependent on the "finest law enforcement agency in the world/The Federal Bureau of Incompetence for your protection is a roll of the dice. You may think that if you see something and do something that something will happen but they may toss the message in the wastebasket and go back to Tweeting with their fbi girlfriend.
That is why the Framers provided for the people to have the right to protect themselves. Other people may or may not protect you and even those who are fully motivated to do so may be minutes or hours away.

