But there were 400 other students in the school the day of the shootings. The oldest — fourth graders at the time — are now in high school, and have gradually begun to realize the power they wield by speaking out. And in increasing numbers, Newtown’s teenagers are joining a network of young activists around the country who say they have had enough.

Those students remember the hourslong lockdown on Dec. 14, 2012. They knew something terrible had happened, but they did not know exactly what. They did not know if they were safe, or if they would see their families again.

Last fall, Natalie joined a handful of students at the first meeting of the Junior Newtown Action Alliance club, run by two seniors, Jackson Mittleman and Tommy Murray. She felt a little uncomfortable, she said — talking about guns and shootings and death is emotionally draining for a teenager who lost her brother in the Sandy Hook massacre. But she stuck with it.

The club had struggled to attract members; at times just a few students showed up to meetings, Jackson and Tommy said. (Tommy Murray’s mother, Po Murray, founded the Newtown Action Alliance after the shootings, and her children have been active in the junior club.)

In February, when 17 people were gunned down at Marjory Stoneman Douglas High School in Parkland, Fla., Natalie said she knew she needed to take decisive steps. “I decided that I just needed to do more,” she said.