"It has to be our generation that says, 'No, we are not going to take this anymore,' " says Hogg, 17

On Feb. 14, Marjory Stoneman Douglas High School lost 17 victims to a tragic mass shooting.

In the days and weeks afterward, as students and families grieved and returned to class, many of the teens who attend the Parkland, Florida, high school have been speaking out about their experiences and demanding there be changes in gun safety legislation so that something like this can never happen again.

As part of a social media initiative called #whatif, photojournalist Jeff Vespa captured some students’ heart-wrenching tales of survival — and their determination to create a future free of gun violence.

Their powerful words and portraits are featured in this week’s issue of PEOPLE, on newsstands Friday.

Watch more Voices of Parkland Survivors on PeopleTV.com, or download the PeopleTV app on your favorite mobile or connected TV device.

During the shooting, 17-year-old senior David Hogg hid in a closet with 65 other classmates who survived.

“It was around that time that I decided I would start recording video so that if I was left in that classroom and all of our 65 souls were left on that classroom floor, our voices would echo on,” he says in the video above.

Since the massacre, Hogg has focused on a mission to stand up for the 17 lives lost last month and push for anti-gun violence measures.

“It has to be our generation that says, ‘No, we are not going to take this anymore,’ ” he says. “Now is our one and only chance to stand up, because if we don’t do it now, when?”

The teens have planned the March For Our Lives in Washington, D.C., on March 24 to demand changes in gun legislation and already the event has expanded to include more than 400 related demonstrations in cities across the globe.