Updated at 1:30 p.m. ET
At South High School in Columbus, Ohio, students stepped outside in frigid weather and said 17 names, releasing a balloon for each one.
In Orange County, Fla., 17 empty desks sat in the Wekiva High School courtyard. Students sang — "Heal the world, make it a better place."
In New York City, hundreds of students from LaGuardia High School walked into the street and sat in silence for 17 minutes.
In Helena, Mont., more than 200 students gathered outdoors and shared stories and details about 17 lives..
Across the country, students are walking out of class for 17 minutes, one for each victim who died at the shooting in Parkland, Fla., exactly one month ago.
But the day of activism is not just limited to students stepping out of their schools.
In Massachusetts, where more than a foot of snow fell on the state on Tuesday, students are preparing to rally at the statehouse and call for change.
Fifth-grade students in Akron, Ohio, who were studying the civil rights movement in class, are organizing a sidewalk protest, The Associated Press reports. At an elementary school in Virginia, students prepared a handwritten folder with information for the press.
In Silver Spring, Md., a long line of sign-holding students walk down a major street, with a police escort blocking traffic.
And outside the White House, protesters chant: "Hey hey, ho ho! The NRA has got to go!"
According to EMPOWER, the youth branch of the Women's March, there are more than 3,130 school walkouts scheduled across the country, as NPR's Adrienne St. Clair reports.
The national organizers called for a 17-minute walkout at 10 a.m. local time in every time zone.
But the actual details of the protest vary from school to school.
Some planned marches earlier in the day. Adrienne spoke to students at Centennial High School in Idaho who scheduled their walkout for 9:28 a.m., when the bell rings, rather than 10 a.m. "This will allow students to walk out in between classes, rather than getting up in the middle of a class," Adrienne writes. "[Student body president Tommy] Munroe said some students may be too scared to leave if they are in a class with a teacher who doesn't support the march, and so may not have an opportunity to participate."
On the other hand, in Providence, R.I., student activists pushed the protest later, to 12:45 p.m., because "students aren't allowed back into school once we walked out," Dorbor Tarley explains on Facebook.
And some schools are going far beyond just a 17-minute walkout.
Students in some areas have organized marches, letter-writing campaigns and rallies with speakers, taking up part or all of the school day.
Various school districts also face different responses from administrators. Some have told students they won't punish walkout participants. Others emphasize that normal school rules are still in place, and leaving class or campus without permission will result in disciplinary action.
Some administrators are promoting alternative forms of protest, like a moment of silence, or finding ways to incorporate the protest into a lesson plan.
Meanwhile, the ACLU is working to educate students about their rights. Schools can discipline students for walking out of class, even for a political protest, the group notes.
"But what they can't do is discipline you more harshly because of the political nature of or the message behind your action," the ACLU writes. "The exact punishment you could face will vary by your state, school district, and school. Find out more by reading the policies of your school and school district."
Meanwhile, students in the Northeast are facing another challenge: the weather.
A northeaster dropped inches of snow on Tuesday, prompting some schools to close and disrupting walkout plans — although not always completely halting protests.
Here are other scenes from walkouts and protests across the country:
In Charleston, S.C., students walked out of class but remained indoors, Victoria Hansen of South Carolina Public Radio reports. They "stayed out of public view for fear protesting gun violence would make them targets of more violence, " she writes.
In Washington, D.C., students from Fairfax County's Thomas Jefferson High School traveled to the U.S. Capitol and sat on the steps, NPR's Brakkton Booker reports, as a "sea of young protesters" gathered in front of the Capitol chanting, "enough is enough" and "books not bullets."
At Boston University, medical students students and professors gathered for a walkout with a slightly different demand — they called not just for gun control, but for research into gun violence. (As NPR's Rebecca Hersher put it earlier this month, scientists have objected for decades to "a profound, and purposeful, lack of federal funding for gun research.")
In the Bronx, students marched to their council member's office, chanting, "They say get back, we say fight back," Danny Lewis of WNYC reports.
In Hillsboro High School in Nashville, Tenn., students chanted "not one more" and "this is what democracy looks like" after their demonstration outside their school, Julieta Martinelli of member station WPLN reports.
In Southwest Detroit, a school walkout at Western International High School included remarks in three languages — English, Spanish and Arabic — member station WDET reports. "We really need to talk about gun violence here in Detroit, because that's something we've normalized over the last couple of years," senior Alondra Alvarez told the station.
At West Liberty-Salem High School in Ohio, the site of a school shooting last year, a small group of students joined the nationwide protest despite being warned they would be punished. Superintendent Kraig Hissong said students who walk out "will receive disciplinary action" and "the absence will be counted as unexcused." Still, 10 students left the school as supporters cheered from across the street, The Associated Press reports.
At Flathead High School in Kalispell, Mont., students similarly showed up for a moment of silence "despite pressure from school administration not to leave their classrooms," Nicole Miller from KAJ News, a local TV station, reports.
At Rapid City Central High School in South Dakota, students gathered in an auditorium where organizers told the crowd, "This is not a protest. This is a walkout," Chynna Lockett of South Dakota Public Broadcasting reports. Then students signed letters to the president expressing opposition to gun violence.
In Draper, Utah, students gathered outdoors with mountains as a dramatic backdrop. One student at Corner Canyon High School told KUER's Lee Hale, "This shooting made me an activist."
In Colorado Springs, the walkout prompted "dynamic conversations," Ali Budner of KRCC reports. Teachers, students and community members gathered outside the school in a ring — while some students "stayed inside to protest the walkout," Budner says.
In Davenport, Iowa, a Twitter account for the local walkout said the students there "are not fighting the 2nd amendment or pushing gun control at all, we are advocating for more [active shooter response] drills, teacher and counselor resources to treat those with mental health issues."
In Chicago and the surrounding suburbs, about 100 schools had walkouts, member station WBEZ reports. The station has collected accounts from students on their website. WBEZ's Miles Bryan reports that one school, in "conservative Plainfield," is giving students who walk out a choice: "either have a meeting with lawmakers to learn about political process— or get one hour detention."
In San Diego, at Patrick Henry High, students held up signs — including one reading, "we go to school to learn, not to die," reports Megan Burks of KPBS News. One student speaker, expanding beyond gun control, urged students to "look up from your cell phones. Look up from your AP tests. Look up from the soccer field. Look up at each other."
And at Marjory Stoneman Douglas High School — the Parkland, Fla., school where last month's shooting took place — students gathered on the football field for a group hug, the AP writes.