Three students, three families ripped apart. One student is dead, one critically wounded and the other awaits trial.

Yet, at a recent community meeting with Mayor de Blasio, Chancellor Carmen Fariña and what was termed “parent leadership,” the horrific incident involving a fatal stabbing and a critical wounding inside a Bronx school of two students by another student, who claimed he’d been bullied by the two for a long time, never touched on a core question: How did a knife get into a classroom?

The mayor and chancellor did not seem particularly interested in answering that, nor in supporting proven ways to improve school safety: adding metal detectors and removing barriers to intervention by school safety agents.

Instead, the suggestions ranged from hiring more guidance counselors specializing in bullyism, to training teachers and other school personnel to be better able to prevent bullying, such as having school safety agents “smile as they enter the school building.”

Despite a report that weapon seizures in schools have spiked by 48 percent from July 1 through Sept. 30 compared with the same time period last year, the mayor stated that “violent crime in public schools is down by 18 percent; bullying is taken seriously and cannot be ignored or tolerated in public schools.” He pledged more funds to combat it and will seek federal assistance so that “every school will have access to mental-health resources.”

Missing was any question of why a knife was able to get into the school in the first place. We now know that a second knife was found in the same classroom of the same school a day later.

A smile wouldn’t prevent that. But metal detectors would. They should be installed in all public high schools. Right now, there are only 81 metal detectors in over 1,000 school buildings.

And, de Blasio, Fariña and the state chapter of the ACLU have been advocating reducing the use of metal detectors based on the theory that they create a punitive environment and impede learning.

Then there’s the way the city ties the hands of school safety agents. City Hall’s task force on school safety declared that “Department of Education personnel shall not request the intervention of school safety personnel when responding to the following normative child and adolescent behaviors, absent a real and immediate threat of serious injury to a member of the school community,” listing “disorderly conduct . . . use of racial or other slurs . . . and bullyism.”

School safety agents shouldn’t be handcuffed. They need realistic guidelines and equipment to protect students and staff. Increased use of “warning cards” issued for possession of marijuana and the broad category of “disorderly conduct” doesn’t deter bad behavior. Changing the definition of a crime doesn’t make the crime go away.

This tragedy could have been avoided with metal detectors, and think that if parents and students of that Bronx school and schools throughout the city were asked, they would overwhelmingly agree. Guidance counselors, sensitivity training and smiling faces greeting students at the door are no substitute for detecting and keeping a potentially deadly weapon from getting into the school building.

Yes, it’s essential to address the root cause of why someone would want to bring a weapon into school, but we must first address how someone can get a weapon into that school in the first place.

Metal detectors save lives. But if the mayor and chancellor are more concerned with hurt feelings and political optics, they should devise another solution that is realistic for the world as it is, not as we wish it were. The carnage must stop before the cures are found.

Saving children’s lives must be our first priority. Just ask three grieving families.

This column was originally published in the New York Post