Less than two weeks ago, Harris T. Jaffe spent two-and-a-half hours hiding in a closet during the Feb. 14 shooting at Marjory Stoneman Douglas High School in Parkland, Fla., wondering if the shooter, Nikolas Cruz, 19, would find him. On Sunday, the 16-year-old was behind a podium at Temple B'nai Abraham in Livingston, calling for legislative action and telling more than 2,300 people his story of survival in a massacre that left 17 of his classmates and teachers dead.