Popular school security strategies have not stopped mass shootings.

Armed school police officers. Lockdown drills. High-tech apps for monitoring bullying and students’ social media posts.

Like many school systems across the country, the school district in Uvalde, Texas, put in place a plethora of recommended safety practices meant, in part, to deter school shootings. But they were of little use on Tuesday, when a gunman killed 19 children and two teachers at Robb Elementary School.

The district’s detailed safety plan illustrates that despite the widespread “hardening” of schools over the past two decades, mass shootings continue on with sickening frequency.

“These security measures are not effective,” said Jagdish Khubchandani, a professor of public health at New Mexico State University who has studied school violence. “And they are not catching up to the ease of access with which people are acquiring guns in the pandemic. All records are being broken in gun sales.”

In Uvalde, a district of 4,000 students, the school district police department included six officers, one of whom was involved in the police response. But there are questions about how the city and school police officers had handled the gunman.

Texas districts like Uvalde have invested heavily in school policing and other security measures in recent years. Texas responded to the 2018 mass shooting at Santa Fe High School with $100 million in school safety funding. In Uvalde, before the shooting, the city’s S.W.A.T. team visited all district schools in “full tactical uniforms,” according to the police department’s Facebook page.

But there is little evidence nationally that the dollars poured into these kinds of prevention measures have decreased gun violence in schools, according to a 2019 study by Professor Khubchandani.

Instead, he wrote, they may be proffering “a false sense of security.”

Social-emotional strategies, including anti-bullying initiatives, also do not appear to forestall senseless tragedy. Uvalde had counselors and social workers available. Threat-assessment teams at each of the district’s schools were on the lookout for warning signs of suicide, according to the district’s safety plan.

Combating bullying was a special focus. The district website displayed the winners of a recent bilingual bullying-prevention poster contest. “Kindness takes courage!” one child wrote.

The district used software called Social Sentinel, which monitors students’ social media posts for threats, and an app called STOPit, which allows anonymous reports of bullying.

These, too, are common practices.

Ron Avi Astor, an expert on school violence at the University of California, Los Angeles, argued that while social-emotional supports have improved school climate broadly, those strategies — as well as the presence of campus police — have been insufficient in preventing suicidal, often ideological young men from accessing guns and carrying out attacks intended to draw fame.

The focus should be on referring high-risk individuals to mental health treatment while preventing them from buying or owning guns, he said.

“We have to start talking about shooters and shootings differently,” he added.

In the hours and days after the tragedy in Uvalde, many policymakers leaned on a familiar response, adding more policing. Officials in Georgia and Virginia deployed additional officers to schools as a precaution. And Senator Ted Cruz of Texas suggested putting more armed police in schools.

At Uvalde, the actions of local law enforcement are under scrutiny. An onlooker told The Times that officers remained outside the building for some time while the gunman was inside, and that parents urged police to storm the school sooner. The gunman gained access to a classroom and reportedly barricaded there for up to an hour. That classroom is where all the fatalities occurred, according to officials.

In an interview, Mo Canady, executive director of the National Association of School Resource Officers, said his organization had trained several Uvalde school officers over the course of four years and that they were typically based at secondary schools, not elementary schools. He warned against jumping to conclusions about officers’ actions.

Storming a building too quickly could allow an active shooter to escape, he said. And while capturing or killing an active shooter is “Plan A,” he said, containing them to a particular space can be an effective “Plan B” to lessen the carnage.

School policing exploded in popularity after the 1999 shooting at Columbine High School in Littleton, Colo., when Congress began providing federal dollars for campus officers. Nationally, 19 percent of elementary school students, 45 percent of middle schoolers and 67 percent of high school students attend a school with a campus police officer, according to a 2018 report from the Urban Institute.

But when the Congressional Research Service looked at the effectiveness of school policing in the wake of the 2012 mass shooting at Sandy Hook Elementary School in Newtown, Conn., it concluded there was little evidence showing that the presence of officers affected crime rates.

Armed school officers have been present at some of the most infamous school massacres and were not able to stop those events. The officer on duty at Marjory Stoneman Douglas High School in Parkland, Fla., in 2018 has been accused of hiding during the shooting that killed 17 people.

During the Columbine shooting, a school resource officer shot at the gunman but missed.

School policing is also divisive, in part because students of color are disproportionately referred to law enforcement, even for routine misbehavior.

But Mr. Canady said that school officers had prevented many instances of violence that do not gain broad attention. He pointed to a National Policing Institute database that showed 120 cases of averted school violence between 2018 and 2020.

Almost every school in the United States holds lockdown drills, and that was true in Uvalde. While some survivors of last year’s shooting at Oxford High School in Michigan credited the trainings with helping them quickly escape the building, there is little evidence that the drills prevent violence — and lots of concern from parents, educators and mental health experts that they cause fear and anxiety for children.

There are some simple, inexpensive measures that are protective, according to those who have studied school shootings. One of them is keeping classroom doors locked, which was a district requirement in Uvalde.

But it was not clear whether that practice was followed at Robb Elementary on the day of the shooting, when individuals were reportedly streaming in and out of the building for an awards ceremony.

The school had “perimeter fencing” designed to restrict access to the campus, according to the district. The safety plan also described the use of the Raptor Visitor Management System, which scans visitor IDs and checks them against sex offender registries and lists of noncustodial parents.

At a news conference in Uvalde on Wednesday, the lieutenant governor of Texas, Dan Patrick, praised the district’s security measures but suggested an area of improvement could be limiting schools to only a single entrance. And in the wake of the shooting, New York City, the nation’s largest school district, said it would consider locking school doors after students arrive for the day. Los Angeles said it would reduce points of entry into schools.

But Professor Khubchandani questioned whether any of these measures would stop a committed killer with access to weapons.

“It’s like medication for heart attacks while continuing to eat bad instead of eating healthy,” he said. “You prevent this from happening or you don’t.”