Samantha Haviland, who survived the mass shooting in Columbine High School nearly 20 years ago in Littleton, Colo., talks about the political actions by students in the wake of the massacre at a Parkland, Fla., high school. —David Zalubowski/AP Denver Patrick Neville was outside, sneaking off to smoke with friends, and avoided the outburst of gunfire at Columbine High School nearly two decades ago, but he did not dodge the heartbreak. A close friend died, and the anguish in his father’s eyes is seared in Neville’s memory. Samantha Haviland was fundraising in the cafeteria and froze, uncomprehending, at the sound of screams just outside the window. Trance-like, she and others fled the room, then pressed against a wall of lockers, windows shot out down the hall. She, too, lost a close friend. The horror of April 20, 1999—13 died when two student gunmen attacked the suburban Denver school—changed Neville and Haviland’s lives in different ways but inspired both to take action...