"My school is being shot up and I am locked inside."

The scared student's tweet was jarring, but what really grabbed Britnee Webb as she scrolled through Twitter were the accompanying photos, taken from the floor and shared live as students hid along a wall and under desks at their Florida school.

Webb flashed back 11 years, remembering how it felt to crouch beneath a table, to wait for police for what felt like forever, hoping the gunshots didn't get closer. Now this boy would know that feeling too. She wanted him to know he wasn't alone.

"I've survived this," she tweeted to him, a social-med...