On a snowy day in mid-January, Sarah DeSouza was sitting in class at the Packer Collegiate Institute, a private school in Brooklyn, when a voice came over the loudspeaker and announced that the school was going into lockdown. DeSouza, a biracial senior, had grown up in Brooklyn, where her parents work in construction. She was taking a science-research class that had paired her with a laboratory at Mount Sinai to do research on diabetes, and she was listening to another student present his work when the lockdown began. The students filed into a storage closet lined with shelves of chemicals and fume hoods for working with noxious substances. The teacher locked the door and turned off the lights. DeSouza has been doing lockdown drills since she was in the seventh grade, when, after the massacre at Sandy Hook Elementary School, teachers taught her to crouch under a desk if a gunman entered the building. “Obviously, I can’t remember when fire drills were instituted,” she said. “But I remember that very moment. It’s so chilling.” Even with so much practice, as she stood in the closet she grew uneasy. “You immediately think, Oh, my God, what's going on?” she said. “Is this a drill? Is this the real deal? You don’t know. You’re scared. You’re hiding in the dark. You can’t talk to your friends.”

A month later, Nikolas Cruz walked into Marjory Stoneman Douglas, his old high school, in Parkland, Florida, and killed seventeen people with an AR-15. DeSouza found out about the shooting from her mother, who drove her home after school. “It was unfathomable,” she said. “I couldn’t believe this had happened to people my age.” Her school sent Marjory Stoneman Douglas a banner that read “We Stand with You,” but, for DeSouza, it wasn’t enough. She watched survivors of the shooting confront lawmakers and N.R.A. leaders on national television. “It inspired me to do something to contribute,” she said. “And to let them know their voices are being heard.”

Further Reading New Yorker writers respond to the Parkland school shooting.

The Parkland students also inspired a high-school senior in California named Winter Minisee, who hoped to provoke similar action across the country. Minisee helps lead the youth arm of the Women’s March, along with five student leaders and one “adult ally.” During the Parkland shooting, she watched videos of the attack that students posted on Snapchat. “Seeing the confusion and the grief at the same moment as them was very galvanizing,” she told me. “It was also very emotional.” Her group decided to organize a nationwide protest on March 14th, one month after the shooting, in which students would leave class for seventeen minutes, one minute for each victim. Minisee helped assemble a team—several of whom had personally been victims of gun violence, including a teacher from Sandy Hook—and they distributed tool kits and paired students with local activists.

DeSouza and others at Packer heard of the movement and began organizing a two-hour rally in downtown Brooklyn. After the majority of students at Packer signed up to attend, the administration cancelled classes for the morning. “I realized that, even if there was class, there wasn’t gonna be no class,” José De Jésus, the head of Packer’s upper school, told me, laughing. That kind of support may be a private-school luxury. At Midwood High School, a public school in Brooklyn, Naomi Giancola wanted to hold a simple memorial for the Parkland victims. She found her principal, Michael McDonnell—whom she described as “politically conservative”—to be unsympathetic. She was allotted exactly seventeen minutes, and was not permitted the use of a microphone. At 1 A.M. on Wednesday, before the walkout, McDonnell sent out an e-mail reminding teachers not to participate and instructing them to close the blinds so that students in class wouldn’t be distracted. (McDonnell denied obstructing the rally. “My politics are mine,” he said. “What—do you want to know how I voted in the election, too?”) Around the country, a handful of districts warned that protesters would be punished. In Needville, Texas, the superintendent, Curtis Rhodes, threatened school-skippers with three-day suspensions, writing, “We will discipline no matter if it is one, 50, or 500 students involved.”

Nevertheless, on Wednesday morning, kids at thousands of schools, in all fifty states, participated in the National School Walkout. At South High School, in Columbus, Ohio, students released seventeen balloons. At Wekiva High School, in Orange County, Florida, they filled the courtyard with seventeen empty desks. In Washington, D.C., lawmakers including Bernie Sanders and Chuck Schumer met protesters in front of the Capitol. The building’s lawn was strewn with seven thousand pairs of children’s shoes—one for each child killed by a firearm since Sandy Hook. Representative John Lewis encouraged activists to remain steadfast. “You know, the N.R.A. don’t like me very much,” he said. “I love you, John Lewis,” a student yelled.

In downtown Brooklyn, DeSouza and hundreds of other Packer students congregated with more than a thousand other students—mostly from nearby private schools—in front of Borough Hall. Minisee and her fellow Women’s March organizers had encouraged protesters to wear bright orange—the color bystanders wear during hunting season, to avoid being shot—and students came out in orange scarves, orange bandannas, and no fewer than three orange fedoras. They carried signs reading “Trump Kills Teens” and “Your gun has more rights than my vagina.” Several simply read “STOP,” including one octagonal metal sign apparently nabbed from a street corner. “Wow, that kid literally stole a stop sign,” a passerby observed. “That’s creative.” Georgia Groome, a freshman at Packer, came to the rally in a bulletproof vest emblazoned with the logos of corporations with financial ties to the N.R.A. “I’ve never been more proud,” her mother told me. The wind was biting, and over time toes grew numb. To warm up the crowd, an organizer with a megaphone led classic protest chants, some of which were evidently dated. An adolescent repeated “We are the ninety-nine per cent” several times before turning to his friend and whispering, “Wait, what does this one mean again?”

The mood before the ceremony was light, and the protesters, being teen-agers, too, took endless selfies. A prankster succumbed to peer pressure and climbed a lamppost, drawing cheers. But when the event began the mood grew serious. DeSouza and her fellow-organizers stood on the steps of Borough Hall and read the names of the Parkland victims, which they followed with a moment of silence. DeSouza was understandably worried about the prospect of getting a thousand teen-agers to shut up—a task any teacher would deem impossible—but when the moment came the silence was real. “I think that set the tone for the rest of the day,” she said.

Afterward, DeSouza ran around carrying a clipboard and muttering rapid-fire orders into a walkie-talkie. She was organizing the lineup of speeches, and she had the slightly harried look of a stage manager. “It’s been really overwhelming to coördinate all of this,” she said. “But this is the most incredible feeling.” Several grownups spoke, including the Brooklyn Borough President, a member of City Council, and the New York City Comptroller, who shouted, “The adults have failed you” to raucous applause. But the stars of the rally were the dozen or so student speakers, who were precocious, well informed, and moving. A few high schoolers approached from the audience and asked to give impromptu speeches. One performed a spoken-word poem. “Some of the best teaching is when students take control of the learning,” De Jésus, the Packer principal, told me. “It was pretty amazing to see.”

Khaja Daniel, a ninth grader at Packer, gave one of the final speeches of the morning. When Daniel was nine, her father, a utility-company worker, was walking out of a bar in Baltimore when a convicted criminal with whom he had a disagreement drew an assault rifle and shot him in the back at point-blank range. “I did not and do not deserve to grow up in a world without my father because my country refuses to protect me,” she said. Daniel had asked DeSouza to stand next to her during the speech, and to take over if she choked up. At one point she began to cry, but DeSouza rubbed her back, and, after wiping her cheeks, Daniel pushed through. “Maintaining that your Second Amendment rights are more important than my right to live is absolute bullshit,” she said. “I refuse to live in a world like this.”