PARKLAND, Fla. — After a gunman turned their high school into a sprawling crime scene last week, three freshman friends leapt into the student movement for tougher gun laws. They rode a bus to the State Capitol and chased down lawmakers. They vowed to march on Washington. They shouted and waved signs saying “Protect Kids” and “Stop Killing the Future.”

But at night, in the blackness that recalls the dark classroom where she hid as a gunman murdered her classmates, Samara Barrack, 15, cannot stop thinking about that afternoon, when she fled through a blood-covered hallway. Samantha Deitsch, also 15, grieves a friend from journalism class. Aria Siccone, 14, who walked past the bodies of students from her last-period study hall, feels nothing sometimes. Just numbness.

“I keep having flashbacks,” Samara said. “There’s times I want to cry and can’t. There’s times I want to have fun and am hysterical.”

This is the reality that confronts students at Marjory Stoneman Douglas High School when the cameras turn off and the day’s rallies are over. They have won praise for their strength and eloquence on the world’s stage. But even as they raise millions of dollars and plan nationwide rallies, parse the details of assault-weapons laws and spar with politicians and conservative critics, the young survivors of the massacre are struggling with the loss of their friends and educators, and the nightmares that flood back in moments of stillness.