It is the average citizen who will be identifying potential threats. It will be up to law enforcement to investigate and DO THEIR DAMNED JOB!
Whatever system is put in place will be abused. Reporting potential shooters will have it’s own version of “swatting” much like red flag laws are used.
The fact that the carnage can be reduced by making sure are doors locked and stay locked and yet the adults in charge cannot make sure such a simple protocol is followed should tell you that more complicated laws and rules aimed at a miniscule proportion of the population will fail the same as locked door rules. Because people are human.
The real answer is rapid evacuation out the windows and have kids run like jackrabbits and allowing teachers to carry.