As the 1980s yielded to the high-crime early ’90s, “zero tolerance” became a mantra in school districts across the United States. “There was a real concern,” Mr. Holder acknowledged to Retro Report, “that we were just losing control as a society.”

It was an era of near-panic over violence by young people. Fears gave rise to the notion of a generation of “superpredators,” a word that has resurfaced in the current political season, including last week’s presidential debate. It was invoked in the ’90s by, among others, Hillary Clinton, who now renounces its use.

And so, back then, suspensions and arrests began to soar. Local authorities were emboldened by the Gun-Free Schools Act of 1994, a federal law that required states receiving federal education money to expel for at least a year any student found bringing a weapon to class.

But the zero-tolerance net came to be thrown ever wider, ensnaring far more than gun toters, knife wielders and drug dealers. Infractions once deemed the province of school disciplinarians — tardiness, say, or mouthing off to a teacher — often made their way to police blotters. There were eyebrow-arching moments like the arrest of a 12-year-old girl for doodling on her desk with a green marker, of an autistic child who had kicked a trash can, of teenagers who got into fistfights (as teenagers have done probably since Neanderthal days).

To some degree, school administrators were like generals who go to battle relying on tactics from the last war. Zero tolerance kicked into high gear, and stayed there, after youth violence had already entered what would become a steep decline. Homicides involving juvenile offenders, for instance, peaked in 1994, Justice Department figures show. By 2014, their numbers had fallen by two-thirds. Even occasional mass murders in schools, horrifying as they are, have not materially altered the overall pattern of reduced mayhem.

It is not lost on researchers that students expelled, suspended or arrested on charges like disorderly conduct are disproportionately black and Latino, or disabled mentally or physically. In kindergarten to 12th grade, blacks were 3.8 times as likely as whites to receive out-of-school suspensions, according to the United States Department of Education. Youngsters in those grades with disabilities were more than twice as likely as others to be suspended.

Researchers talk about a “school-to-prison pipeline” that runs like this: Young people are suspended from classes for long stretches, or are handed over to the police. As a result, they become prime candidates for quitting school entirely. Dropping out, in turn, makes them less likely to find jobs and more likely to become part of the criminal class.

Perhaps not surprisingly, a sense that school systems and police departments went overboard has begun to take root. An outspoken critic is Steven C. Teske, the chief judge of juvenile court in Clayton County, Ga., just south of Atlanta. Teenagers, Judge Teske has cautioned, will be teenagers.

“Zero tolerance as a philosophy and approach is contrary to the nature of adolescent cognition,” he told a Senate subcommittee in 2012. For all the arrests, suspensions and expulsions that he had observed, “school safety did not improve,” he said. If anything, “the juvenile crime rate in the community significantly increased.”

“These kids lost one of the greatest protective buffers against delinquency — school connectedness,” the judge said.

To foster that connectedness, some schools are shunning harsh punishment in favor of talking things through with rule breakers. They are places like Furr High School in Houston. Its principal, Bertie Simmons, prefers consequences that are “academic,” as with two students who forged a permission slip. Rather than being suspended or put on detention, they were required to write a paper about their offense.

“If you just treat people with kindness, it’s far better than being so punitive,” Ms. Simmons told Retro Report.

No public school system in the country is bigger than New York City’s, with 1.1 million students. It, too, has moved away from harsh discipline as an automatic response. Suspensions in the second half of 2015 were down by one-third from the same period the year before.

At the same time, safety improved. Major crimes — like rape, felony assault, burglary and robbery — were reported at their lowest level since the police started tracking them in 1998.

For many months, the administration of Mayor Bill de Blasio has even raised the possibility of removing metal detectors from some of the scores of school buildings where they are fixtures. Many students regard them as “intrusive and denigrating,” a mayoral panel concluded last year.

But transforming talk into action has been slow. An episode like that of the boy caught trying to slip a gun into school last month is unlikely to dissuade school safety agents and others who insist that the scanners save lives.

Notwithstanding the need for continued vigilance against that sort of lawbreaking, Mr. Holder contends that broad changes are essential. “We have a connection between our school system and the criminal justice system that did not exist before and that I don’t think should exist now,” he said.

CLYDE HABERMAN, a regular contributor to Retro Report, has been a reporter, columnist and editorial writer for The New York Times, where he spent nearly 13 years based in Tokyo, Rome and Jerusalem. Subscribe to our newsletter here and follow us on Twitter @RetroReport.

This article first appeared in The New York Times.