In the summer of 2020, Denver school leaders quickly banished police officers from campuses and directed the funds toward social workers and psychologists. The city, like many across the nation, was roiled by protests against law enforcement after the murder of George Floyd by a Minneapolis police officer.

School board members cited as a major reason reams of data showing that Black students were far more likely than white students to be arrested.

But a spasm of violence affecting Denver city schools this year has brought a sudden reversal. Armed officers were quickly deployed this spring for the final weeks of school. And come fall, school resource officers will return permanently to Denver schools, as fears over student safety have mounted.

In the three years since Mr. Floyd’s death, just as the broader movement to defund the police faltered as crime surged, the push to remove the police from schools has stalled and in many cases reversed amid America’s unrelenting epidemic of gun violence. Communities across the country that had banned school resource officers, from Alexandria, Va., to Pomona, Calif., have changed course. And some larger cities that removed officers, such as Seattle and Washington, are embroiled in contentious debates about bringing them back.

The renewed push is unfolding just as prosecutors in Florida are seeking a criminal conviction for Scot Peterson, the longtime school resource officer in Parkland who waited in a campus alcove instead of confronting the gunman who killed 17 people in 2018. The trial — thought to be the first in the nation against a member of law enforcement for inaction in a school shooting — has raised questions about the duty of campus officers during school violence. Jurors began deliberating on Monday over whether Mr. Peterson was guilty of child neglect, among other charges.

The debate over resource officers also comes after the nation set a record for school shootings in 2022, punctuated by the murder of 19 elementary school students and two teachers in Uvalde, Texas. A CBS News poll conducted after the Uvalde shooting showed that 75 percent of parents of school-age children wanted armed security on their campuses.

The role of school resource officers is as complicated as ever. They serve not just as armed protectors, but also as counselors and disciplinarians, trying to prevent problems before they occur.

The evidence is clear, however, that their presence results in disproportionate suspensions and arrests of Black and Latino students. And the Parkland and Uvalde shootings highlighted failures by the police to stop mass killings, bringing into question how effective they may actually be.

The decision to bring officers back to schools in Denver was driven in large part by parents who mobilized after shootings this year. Luis Garcia, a popular soccer player at East High School, was shot in his car near campus in February. Less than six weeks later, a 17-year-old student at the same school shot and wounded two school administrators; he was later found dead of a self-inflicted gunshot.

“I’m an angry, frustrated parent,” said Dorian Warren, whose son attends East High School. She said she came to the issue “speaking as a woman of color, and as a mother of an African American child.”

While policing results in disproportionate punishments for students of color, Ms. Warren said they were being victimized disproportionately as well.

“You have to do what you have to do to keep these kids safe,” she said. “And I feel like we are gambling every time we don’t.”

Another mother, Heather Lamm, said her son, Jasper, a soccer teammate of Luis’s, had been traumatized not just by the shootings but also by a number of lockdowns at his school over threats of violence that didn’t transpire.

Decades ago, schools relied on local police departments for campus security but typically did not install armed officers on campus. That changed amid crime fears in the 1990s. The 1999 mass shooting at Columbine High School in suburban Denver further convinced school leaders that more campus security was necessary.

Today, districts have a variety of arrangements for employing armed officers. Most are sworn officers assigned to schools by local departments, although some school districts use private security officers and a few have their own police departments.

Despite popular support for armed officers in schools, numerous studies have shown that such policing does little to prevent shootings or gun violence. Ben Fisher, a professor at the University of Wisconsin-Madison who focuses on the intersection of schools and criminal justice, analyzed nearly three dozen studies of school police and wrote that the presence of officers “contributes to increased punishment of students without providing improvements in school safety.”

“I think it’s sort of the American imagination of what police do,” he said. “If there’s a social problem that seems sticky enough, our impulse seems to be to put more police on the job. And so it feels like that’s another thing happening with schools.”

As was the case in Parkland, there is no guarantee that school resource officers will be enough to stop a mass shooter, even though they receive considerable active-shooter instruction. During Mr. Peterson’s trial, prosecutors presented evidence about the training he underwent on mass-shooting scenarios, with video simulations and live actors, and argued that he didn’t follow what he had learned. Mr. Peterson has said that he was unable to determine where the shots were coming from, and that he acted by locking down the school and organizing responding officers.

After Uvalde and Parkland, the duty to confront school shooters has become ever more clear, law enforcement leaders say.

“That’s what we all sign up for,” Chief Ron Thomas of the Denver Police Department said in an interview. “To protect life and property. Beyond the legal obligation, I just think there’s an obligation that we take on as police officers to protect people.”

School police officers say they try to establish relationships with students so tragedies can be averted. “Ninety percent you are being a guardian, 10 percent you are being a warrior,” said Rudy Perez, who worked for 23 years as an officer for the Los Angeles Unified School District, which slashed more than 100 officers in 2020.

Mr. Perez, now the assistant police chief in Golden Valley, Minn., said he believed several shootings were prevented while he was working in Los Angeles schools, by connecting students with services. “And guess what, you never heard about them because it didn’t happen,” he said.

Activists who are opposed to officers in schools say that too often, adolescent misbehavior is criminalized, such as an incident in Tennessee last year where a school resource officer pepper-sprayed and arrested a teenager after he refused to play kickball in gym class. A video of the episode went viral, and students there staged a walkout.

In 2020, the Denver school board unanimously agreed to remove officers from schools, but voted 4 to 3 to reverse that policy this month, after a contentious board meeting.

Tay Anderson, a school board member and parent who was one of the most visible activists during the 2020 racial justice protests in Denver, voted against returning officers to schools, believing that they contributed to the “school-to-prison pipeline.”

“It was rooted in fear,” he said of the board’s decision.

The Denver Police Department and school district are now formalizing an agreement aimed at ensuring that the police stay in their lane. Chief Thomas said that his staff was working with the city’s department of children’s affairs to find alternatives to arrests and citations. And over the summer, school officers will receive specialized training, including lessons on adolescent brain development.

“Kids running down the hallway, screaming, using foul language, that’s a school discipline matter and we’re going to let the schools handle that however they see fit,” Chief Thomas said, adding that his officers would focus only on criminal violations.

“Outside of that,” he continued, “we will certainly serve as a deterrent to an active shooter situation.”