Sunday afternoon, Cameron Kasky is doing push-ups in a park near Marjory Stoneman Douglas High School. Kasky, a junior, says kids like himself are doing something new, demanding a fresh look at America's gun laws.

"The crescendo has hit its point. It's enough and it's over," he says. "I haven't got a shred of doubt that this is going to be our change."

Police say 19-year-old Nicholas Cruz has confessed to murdering 17 people last Wednesday with an AR-15 semi-automatic rifle, which the troubled teen was able to purchase legally. Kasky and most of the other kids in the park survived the attack and now they're pivoting hard trying to create a new national movement.

They've announced plans for a massive rally against school and gun violence in Washington, D.C., on March 24, with smaller rallies and protests in cities around the U.S.

"I just know we must prioritize lives over guns," says Dylan Redshaw, a 17-year-old senior.

Sophie Whitney, age 18 and also a survivor, nods and says, "We can't dwell on the sadness. Of course we're all heartbroken, but we can't let the 17 people die for nothing. We have to make something good out of their death."

At a picnic table in this city park a short drive from their school, the kids have set up a kind of media center. They're fielding calls from news outlets all over the country and also from community organizers who want to help by donating or volunteering. This kind of activism feels really different, compared with past mass shootings.

The kids here say in part it's because the victims are old enough to have a voice. "After what happened in Newtown, those kids were too young to speak out against what happened and to really even maybe even understand what happened," says Chris Grady, age 18, also a senior and a survivor.

"We want to be the voices not only for them but for any student or teacher affected by acts of cowardice like this," he adds.

Another big change after the Parkland attack has been social media. Brendan Duff is a college student who went to school at Marjory Stoneman Douglas High School. He's come home to help manage the new movement's digital campaign. He says the response has been overwhelming, with hundreds of messages per minute pouring in.

"People all over the country want to help. Social media is honestly the best way to reach not only everyone in this country I think, but definitely this generation," Duff says.

This is all happening really fast. Emma Gonzalez, another survivor of Wednesday's attack, went viral nationwide over the weekend after speaking at an anti-gun rally in Fort Lauderdale.

"If you actively do nothing, people continually end up dead, so it's time to start doing something," Gonzalez demanded, sparking a roar of applause.

On this Sunday afternoon, Gonzalez is here in the park helping plan next steps. Her mom Elizabeth Weigard says it's been "terrifying" watching her daughter get caught up in this highly charged moment.

"All you want to do is hold them tight," Weigard says. "Like I don't even want her to go to the bathroom on her own. And she's just like camped out all night with these amazing kids organizing a movement. It's kind of like letting them drive for the first time, which up until a week ago was my biggest fear. You just got to open your arms and let them fly."

These kids know they're flying in the face of massive political opposition, headwinds that include the NRA's staunch opposition to gun control, and the Republican Party's distrust of limiting gun rights. Sophie Whitney, one of the student organizers, says she has a question for those people: "Why is your right to own an AR-15 more important than a kid's right to feel safe?" She quickly answers her own question, saying, "It's not, it is common sense."

Another student, Dylan Redshaw leans in to speak, voice shaking with anger, and says the kids here aren't looking for gun bans. "We just need age restrictions and high-quality, accessible mental health institutions, and higher checks when people are trying to purchase these weapons," he argues.

These kids moved quickly the last couple days to build a social media campaign, while also tapping into the national grassroots network first organized ahead of the Women's March last year. With that help, they think the rally in Washington, D.C. on March 24 will be big.

"We're going to have a place in every major city somewhere where that people all across the country can go to," Stoneman Douglas alum Brendan Duff says. "They want to feel engaged, they want to feel like they're doing something to help. And this is it."

Another student tells NPR this campaign isn't just focused on rallies and social media. It's also about the midterm election. A lot of high school kids are 18 years old or will turn 18 before the November election.

"Our kids are dying and no one is doing anything about it," she says. "Everyone's going to vote."

NPR's Digital News intern Asia Simone Burns produced this story for the Web.

Copyright NPR 2019.