This happened two counties over from where I live. One of the teachers who was present at the time is an old friend from childhood. She did her job, herding the children out of the danger area, but she is very shaken up.
My household includes two elementary-school children, so this hits close to home.
We are very gratified that positive action was taken and no one was killed at the school.
We live in a nation of 330 million people, roughly, with a mental health care system that has been dysfunctional since the 1980s. Dangerously ill people slip through the cracks all the time; beds in secure institutions are scarce and it is hard to commit someone. In some cases, families and friends hide the person's mental state from the public and the authorities or simply "don't want to make waves". Given these circumstances, and the emotional turmoil that is common to teens, it is no surprise this sort of thing will happen from time to time.
Disarming the American public is not a viable answer. Not only are there Constitutional issues, there is the fact that there are more than 300 million guns in circulation already, most never registered. Removing even the majority of them would take a draconian police state and house to house searches, and still would not ensure the disarming of criminals and crazies.
Remember Charlie Hebdo, which happened in France amid strict gun control, and the Norway massacres which happened despite strict gun control. America is not the only place where such things happen.
Certain things have been established as fact about these mass shootings and attempted mass shootings:
The shooter typically targets a vulnerable group, commonly in a "gun free zone".
The shooter is typically stopped only by the arrival of armed opposition.
I'll let you do the math.