They bowed their heads in honor of the dead. They carried signs with messages like “Never Again” and “Am I Next?” They railed against the National Rifle Association and the politicians who support it.

And over and over, they repeated the message: Enough is enough.

In a wave of protests one historian called the largest of its kind in American history, tens of thousands of students walked out of their classrooms on March 14 to demand action on gun violence and school safety.

The demonstrations extended from Maine to Hawaii as students joined the youth-led surge of activism set off by the February 14 massacre at Marjory Stoneman Douglas High School in Parkland, Florida.

“We’re sick of it,” said Maxwell Nardi, a senior at Douglas S. Freeman High School in Henrico, Virginia, just outside Richmond. “We’re going to keep fighting, and we’re not going to stop until Congress finally makes resolute changes.”

Students around the nation left class at 10 a.m. local time for at least 17 minutes—one minute for each of the dead in the Florida shooting. Some led marches or rallied on football fields, while others gathered in school gyms or took a knee in the hallway.

At some schools, hundreds of students poured out. At others, just one or two walked out in defiance of administrators.

They lamented that too many young people have died and that they are tired of going to school afraid they will be killed.

“Enough is enough. People are done with being shot,” said Iris Fosse-Ober, 18, a senior at Washburn High School in Minneapolis.

In joining the protests, the students followed the example set by many of the survivors of the Florida shooting, who have become gun-control activists, leading rallies, lobbying legislators and giving TV interviews. Their efforts helped spur passage last week of a Florida law curbing access to assault rifles by young people.

Another protest against gun violence is scheduled in Washington on March 24, with organizers saying it is expected to draw hundreds of thousands.

But whether the students can make a difference on Capitol Hill remains to be seen.

Some students have issued specific demands for lawmakers, including mandatory background checks for all gun sales and a ban on assault weapons like the one used in the Florida bloodbath.

While administrators and teachers at some schools applauded students for taking a stand—and some joined them—others threatened punishment for missing class.

As the demonstrations unfolded, the NRA responded by posting a photo on Twitter of a black rifle emblazoned with an American flag. The caption: “I’ll control my own guns, thank you.”

The protests took place at schools from the elementary level through college, including some that have witnessed their own mass shootings: About 300 students gathered on a soccer field at Colorado’s Columbine High, while students who experienced the Sandy Hook Elementary School attack in 2012 marched out of Newtown High School in Connecticut.

In the nation’s capital, more than 2,000 high-school age protesters observed 17 minutes of silence while sitting on the ground with their backs turned to the White House. President Donald Trump was out of town.

The students carried signs with messages such as “Our Blood/Your Hands” and “Never Again” and chanted slogans against the NRA.

In New York City, they chanted, “Enough is enough!” In Salt Lake City, the signs read, “Protect kids not guns,” “Fear has no place in school” and “Am I next?”

Marjory Stoneman Douglas High senior David Hogg, who has emerged as one of the leading student activists, livestreamed the walkout at the tragedy-stricken school on his YouTube channel. He said students could not be expected to stay in class while there was work to do to prevent gun violence.

David Farber, a history professor at the University of Kansas who has studied social change movements, said it is too soon to know what effect the protests will have. But he said Wednesday’s walkouts were without a doubt the largest protest led by high school students in the history of the U.S.

“Young people are that social media generation, and it’s easy to mobilize them in a way that it probably hadn’t been even 10 years ago,” Dr. Farber said.

Wednesday’s coordinated protests were loosely organized by Empower, the youth wing of the Women’s March, which brought thousands to Washington last year. The group announced the time and focus on social media, and provided a space where any school’s students could announce their plans.

At Aztec High School in a rural, gun-friendly part of New Mexico where many enjoy hunting and shooting, students avoided gun politics and opted for a ceremony honoring students killed in shootings—including two who died in a December attack at Aztec.

“Our kids sit on both ends of the spectrum, and we have a diverse community when it comes to gun rights and gun control,” Principal Warman Hall said.

In Brimfield, Ohio, 12-year-old Olivia Shane, an avid competitive trap shooter who has owned her own guns since she was about 7, skipped the gun protest held at her school.

“People want to take away our guns and it’s a Second Amendment right of ours,” she said. “If they want to take away our Second Amendment right, why can’t we take away their amendment of freedom of speech?”