A month ago, hundreds of teenagers ran for their lives from the hallways and classrooms of Marjory Stoneman Douglas High School, where 17 students and staff had been shot to death.

On Wednesday, driven by the conviction that they should never have to run from guns again, they walked.

So did their peers. In New York City, in Chicago, in Atlanta and Santa Monica; at Columbine High School and in Newtown, Conn.; and in many more cities and towns, students left school by the hundreds and the thousands at 10 a.m., sometimes in defiance of school authorities, who seemed divided and even flummoxed about how to handle their emptying classrooms.

The first major coordinated action of the student-led movement for gun control marshaled the same elements that had defined it ever since the Parkland shooting: eloquent young voices, equipped with symbolism and social media savvy, riding a resolve as yet untouched by cynicism.

“We have grown up watching more tragedies occur and continuously asking: Why?” said Kaylee Tyner, a 16-year-old junior at Columbine High School outside Denver, where 13 people were killed in 1999, inaugurating, in the public consciousness, the era of school shootings. “Why does this keep happening?”

Even after a year of near continuous protesting — for women, for the environment, for immigrants and more — the emergence of people not even old enough to drive as a political force has been particularly arresting, unsettling a gun control debate that had seemed impervious to other factors.

In Florida, where students from Stoneman Douglas High and other schools had rallied in the state capital, the governor signed a bill last week that raised the minimum age to purchase a firearm to 21 and extended the waiting period to three days.

On a national level, the students have not had the same impact. This week, President Trump abandoned gun control proposals that the Republican-led Congress had never even inched toward supporting.

But, for one day at least, the students commanded the country’s airwaves, Twitter feeds and Snapchat stories.

Principals and superintendents seemed disinclined to stop them. Some were outright supportive, though others warned that students would face disciplinary consequences for leaving school. At many schools, teachers and parents joined in.

Wreathed in symbolism, the walkouts generally lasted for 17 minutes, one for each of the Parkland victims. Two more nationwide protests are set to take place on March 24 and on April 20, the anniversary of the Columbine shooting.

On a soccer field burned yellow by the Colorado sun, Ms. Tyner stood alongside hundreds of her fellow students, who waved signs — “This is our future,” one said — and released red, white and blue balloons.

Yet in many places, for many students, Wednesday was just Wednesday, and class went on. Even at Columbine, the embrace of the gun control movement was not universal.

“People say it’s all about gun control, it’s all about, ‘We should ban guns,’” said Caleb Conrad, 16, a junior, who stayed in class. “But that’s not the real issue here. The real issue is the people who are doing it.”

In the one-school rural community of Potosi, Wis., no student group had organized a protest. After a handful of students expressed some interest, the school decided to hold an assembly at 10 a.m. to talk about school safety measures and the value of being kind to one another.

At 10 a.m., one student, a female freshman, left the building alone.

Throughout the assembly, she sat by herself outside, by a flagpole, for 17 minutes. She appeared to be praying, said the principal, Mike Uppena, adding that she was not in trouble for leaving.

Officials in Lafayette Parish, La., initially said that students could participate in the day’s events, believing that it was appropriate to honor the Florida victims. But when it became clear there was a political motive to the walkout, a torrent of complaints from the local community led the school board to adopt a new plan: a minute of silence.

Dozens of students walked out anyway.

Out of Class and Into the Streets

In some places, demonstrators chanted and held signs. At other schools, students stood in silence. In Atlanta, some students took a knee.

Thousands of New York City students converged on central locations — Columbus Circle, Battery Park, Brooklyn Borough Hall, Lincoln Center.

Gov. Andrew M. Cuomo, a Democrat, stretched out on the sidewalk as part of a “die-in” with students in Zuccotti Park in Lower Manhattan, the former home of the Occupy Wall Street protests.

Hundreds sat in the middle of West 62nd Street for several minutes before rising to their feet and shouting, “No more violence.” A cry of “Trump Tower!” sent dozens of protesters marching toward the Trump International Hotel and Tower across Broadway. Onlookers gave them fist-bumps.

In Washington, thousands left their classrooms in the city and its suburbs and marched to the Capitol steps, their high-pitched voices battling against the stiff wind: “Hey-hey, ho-ho, the N.R.A. has got to go!” One sign said: “Fix This, Before I Text My Mom from Under A Desk.”

Members of Congress, overwhelmingly Democratic, emerged from the Capitol to meet them. Trailed by aides and cameras, some legislators high-fived the children in the front rows, others took selfies, and nearly all soon learned that the young protesters had no idea who they were.

Except, of course, for “BERNIE SANDERS!” which the protesters screamed at the Vermont senator, as well at some other white-haired, bespectacled legislators.

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Asked by reporters about the walkouts, Raj Shah, Mr. Trump’s deputy press secretary, said the president “shares the students’ concerns about school safety” and cited his support for mental health and background check improvements.

As the hours passed, the walkouts moved west across the country.

“It’s 10 o’clock,” said a man on the intercom at Perspectives Charter Schools on Chicago’s South Side. With that, hundreds of students streamed out of their classrooms and into the neighborhood, marching past modest brick homes, a Walgreens and multiple churches.

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Several current and former Perspectives students have been killed in recent years, the school president said.

“You see different types of violence going on,” said Armaria Broyles, a junior who helped lead the walkout and whose older brother was killed in a shooting. “We all want a good community and we all want to make a change.”

At Santa Monica High School in Southern California, teachers guided hundreds of students to the football field. It felt like a cross between a political rally and pep rally, with dozens of students wearing orange T-shirts, the color of the gun control movement, and #neveragain scrawled onto their arms in black eyeliner.

“It is our duty to win,” Roger Gawne, a freshman and one of the protest organizers, yelled to the crowd.

Staying Silent, for the Opposite Reason

Although the walkouts commanded attention on cable television and social media for much of Wednesday, it also was clear that many students did not participate, especially in rural and conservative areas where gun control is not popular.

At Bartlesville High School in Bartlesville, Okla., where hundreds of students walked out of class last month to protest cuts in state education funding, nothing at all happened at 10 a.m.

“I haven’t heard a word about it,” the principal, LaDonna Chancellor, said of the gun protest.

In Iowa, Russell Reiter, superintendent of the Oskaloosa Community School District, suggested that temperatures below 40 degrees may have encouraged students to stay indoors, but he also said that “students here are just not interested in what is going on in bigger cities.”

There was opposition even in liberal Santa Monica. Just after the organizers of the walkout there read the names of the Parkland victims, another student went on stage, grabbed the microphone and shouted “Support the Second Amendment!” before he was called off by administrators.

‘We Need More Than Just 17 Minutes’

Some of the day’s most poignant demonstrations happened at schools whose names are now synonymous with shootings.

Watched by a phalanx of reporters, camera operators and supporters, hundreds of students crowded onto the football field at Stoneman Douglas High shortly after 10 a.m.

A month after the Feb. 14 shooting, notes of condolence, fading flowers and stuffed toys, damp from recent rain, still lay on the grass outside the school and affixed to metal fences.

The walkout was allowed by the school, but several students said they were warned that they would not be permitted back onto the campus for the day if they left school grounds. Despite the warning, a couple of hundred students marched to a nearby park for another demonstration.

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“We need more than just 17 minutes,” Nicolle Montgomerie, 17, a junior, said as she walked toward the park.

An email from the school soon went out telling students they could return.

In Newtown, Conn., where 26 people were killed at Sandy Hook Elementary School in 2012, hundreds of students at Newtown High School gathered in a parking lot near the football field. Two hours later, it was Columbine’s turn.