It would have required one of those teachers to have decided "This is my moment in history to make a difference," - - - and die! Then the other teacher would have to realize that the rifle required reloading and rushed him before he could reload and kill her, assuming that she did not try to herd over the children like a mother hen in a vain or futile effort to protect them.
As is oft times said "No plan survives first contact with the enemy."