In an unprecedented show of unity and political solidarity, waves of students marched out of class Wednesday to demand stricter gun laws and an end to school massacres.
The National School Walkout started at 10 a.m. ET and will continue across the country at 10 a.m. in each time zone. The protest was sparked by last month's school massacre in Parkland, Florida, and fueled by years of anger about what many say are inadequate gun laws.
Those participating have three main demands for Congress:
-- Ban assault weapons;
-- Require universal background checks before gun sales;
-- Pass a gun violence restraining order law that would allow courts to disarm people who display warning signs of violent behavior.
Students planned to stay outside for at least 17 minutes -- one minute for each of the 17 people killed at Parkland's Marjory Stoneman Douglas High School exactly one month ago
Students at Stoneman Douglas said they were overwhelmed by the nationwide support.
There's a "sea of people everywhere. You can barely see the ground," Stoneman Douglas student Sam Zeif said. "It really shows us we're not alone."
In Hoboken, New Jersey, students chanted, "I am a bullet-free zone," and some held signs that read, "Chalk not Glocks!"
Hours before the walkout officially began, students from Montgomery Blair High School in Maryland, escorted by slow-moving police cars, marched to a Metro station, where they boarded a train to the White House.
By 10 a.m., students covered the entire front of the White House, chanting, "We want change!"
"History has its EYES on you," one student's sign read, though President Donald Trump wasn't scheduled to be at the White House at that time.
In New York, students warned lawmakers that they are the future -- and will soon be old enough to vote officials out of office, unless they pass tougher laws.
"We are the change!" they chanted.
Walkout goes global
From Israel to Tanzania, students across the globe also left their classrooms Wednesday in solidarity with the American students' movement. In some places, students talked with teachers about "how lucky they are" that guns aren't a part of their everyday lives.
Eduard Štrébl, a senior at Walworth Barbour American International School in Israel, organized the walkout on his campus.
"I'm from Prague, Czech (Republic), and I'm not American," he said. "But to see an epidemic of school shootings in a developed country when it's so easy to limit such things, to see that there is nothing being done against that, that inspired me to organize the walkout here," he said.
Not just about school massacres
Organizers from the Women's March youth branch started calling for students across the US to walk out of class on March 14, to pressure lawmakers to act on gun control. In addition to walkouts, students across the country planned rallies, marches and sit-ins -- some in open defiance of their school districts.
Participants say they want to make sure that calls for change in the wake of Parkland take into account the broader context of gun violence in the United States.
For D'Angelo McDade, a senior at North Lawndale College Prep High School in Chicago, gun violence is personal -- but not because of a shooting at school.
He was shot in the thigh as he sat on his front porch last summer, leaving bullet fragments in his body, he said. As soon as he was released from the hospital, he started talking to his principal about ways to fight gun violence. On Wednesday, he planned to lead more than half of the school's 600 students on a walkout to converge with teens from other schools.
"Many of our community members and young adults have established a sense of hopelessness and normalized the suffering that comes with gun violence," he said. "But they're ready to see a change."
Penalties for walking out
Some school districts have said they will discipline students who participate in the walkouts.
Students who leave classes in New Richmond, Ohio, for instance, will receive an "unexcused tardy," the district said. For students in Montgomery County, Maryland, walking out will count as an unexcused absence.
In the Atlanta suburb of Cobb County, Georgia, the school district said it will take disciplinary action -- ranging from Saturday school to five days' suspension, per district guidelines -- against students who walk out, citing safety concerns.
The prospect has deterred some students, but not all of them, Pope High School senior Kara Litwin said.
"Change never happens without backlash," she said Tuesday. "This is a movement, this is not simply a moment, and this is only the first step in our long process."
Outside Walton High School in Cobb County, parents stood Wednesday morning with signs reading, "Children Over Guns" and "We Demand Action!"
Growing up in the shadow of gun violence
Students who planned to participate in the walkouts said they feel their generation has been profoundly shaped by the specter of gun violence. By raising their voices, they hope they will be the last kids to grow up with metal detectors and active shooter drills.
Sam Craig of Littleton, Colorado, was not alive during the 1999 Columbine High School shooting that put his hometown on the map. But the tragedy shaped his life.
He grew up with school lockdown drills performed in the name of Columbine. His internship at Denver Zoo includes live shooter drills that include references to Columbine. He knows a teacher who was at Columbine during the shooting, who shares his view that school staff should not be armed, he said.
They survived a school shooting. Now, activism feels more urgent than classes.
But the Chatfield High School junior said the community is stronger because of the shooting. People look out for each other because they don't want anyone to feel "pushed to the point of no return" like the Columbine shooters, he said.
Each year, the town comes together on the anniversary for a day of service, he said. "We try to find that balance to make our community more connected and loving," said Craig, who is organizing the walkout at his school. Abigail Orton, a junior at Columbine High School, said she was inspired to take action on Wednesday by the quick progress of the Parkland students. "I am absolutely amazed at the amount that they've already accomplished, getting their voices out there and being able to speak on this so recently after the event, and to be able to use their status to start bringing about change," she said.
"I'm honored to be able to call this my generation and to be part of this movement." Scenes too familiar
Jackson Mittleman was an 11-year-old sixth-grader when a gunman killed 20 first-graders and six educators at Sandy Hook Elementary, two miles from his school -- a tragedy that changed the course of his life.
Now 16, he's a gun control advocate who planned to join the Wednesday's school walkout.
"A message we're trying to send to Parkland is we stand behind them," said Jackson, co-chair of the Jr. Newtown Action Alliance, who is organizing the walkout at Newtown High School. "We are motivated and we are fired up to push as hard as they push and fight as long as they fight."
When Mittleman opened a news alert on his phone on Valentine's Day and saw the tragically familiar image of students with their hands raised, fleeing a shooting, his heart ached for the students of Marjory Stoneman Douglas High.
He saw yet another community joining what he calls a family "no one wants to be a part of."
He asked himself, "Is it ever going to stop?"
CNN's Bill Kirkos, Brad Parks, Chuck Johnston, Amanda Watts, Gianluca Mezzofiore and Leslie Holland contributed to this report.
