By Emanuella Grinberg, CNN

(CNN) -- Jackson Mittleman opened a news alert on his phone on Valentine's Day, and saw a tragically familiar image: Students with their hands raised, fleeing a shooting.

It brought him back to December 14, 2012, when similar images from his hometown of Newtown, Connecticut, were broadcast around the country. On that day, his community joined what he calls a family "no one wants to be a part of."

Now the people of Parkland, Florida were joining it, too. His heart ached for the students of Marjory Stoneman Douglas High as he thought about what they were going through, and what lay ahead. "Is it ever going to stop?" he asked himself.

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's joining the national school walkout on Wednesday that's part memorial and part protest.

"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."

A national day of protest

Last month, organizers from the Women's March youth branch started calling for students across the country to walk out of class for 17 minutes -- one for each victim who died in the Parkland shooting -- on March 14, to pressure lawmakers to act on gun control.

Now, in addition to walkouts, students across the country are planning 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 in the summer of 2017, 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 plans 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."

Security or criminalization?

In response to the Parkland shooting, the White House has proposed that some school personnel be provided with "rigorous" firearms training.

Student organizers behind the day of action said in a conference call Monday that they feared introducing more guns or police into schools could turn them into prisons, with dangerous consequences for students of color.

"Yes, we are standing in solidarity with the youth from the mass shooting, but we also know that the repercussions of what's going to happen next is going to land on black and brown students within low income communities," said Keno Walker with Power U Center for Social Change in Miami, a youth organization that is helping local high school students organize walkouts.

Several student activists said that having more police and enhanced security measures in schools would make them feel less safe.

Andrea Colon, a senior at Rockaway Park High School for Environmental Sustainability in New York, said her school already has metal detectors and police officers. The impact is "dehumanizing," she said.

"It creates this sense of criminalization that no one really wants to feel," she said. "School is supposed to be a place where you go and feel safe, you feel supported, and that's not how you feel when you have to go through metal detectors, and you're patted down because you have too many bobby pins in your hair or because you didn't take your belt off and you have to be wanded."

"Instead of arming teachers with guns or adding more 'safety' or police, we want better school facilities, more mental health support, more school supplies and safety that does not involve the police," said Ilene Orgaz, a junior at KIPP Denver Collegiate in Colorado.

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 example, 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."

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 has 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.

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."