The references provided by CM are most informative - recommended reading

Here is a key paragraph from the FBI report:

The study identified 21 (13.1%) of 160 incidents where unarmed c
itizens made the selfless
and deeply personal choices to face the danger of an active shooter. In those instances,
the citizens safely and successfully disrupted the shootings. In 11 of those 21 incidents,
unarmed principals, teachers, other school staff and students confronted the shooters to
end the threat. In 10 incidents, citizens, working or shopping when the shootings began,
successfully restrained shooters until police could arrive. And in 6 other incidents, armed
off-duty police officers, citizens, and security guards risked their lives to successfully end
the threat. These actions likely saved the lives of students and others present