With stunning poise and steely confidence, Cameron Kasky, a junior at Marjory Stoneman Douglas High School, confronted U.S. Sen. Marco Rubio, R-Fla., live on CNN on Wednesday evening. “Can you tell me right now that you will not accept a single donation from the (National Rifle Association)?” he demanded. In the arena where the town hall event was held, the crowd of some 7,000 — most of them connected to the school community that was shattered when a gunman killed 17 during a Feb. 14 rampage — stood and cheered for more than 20 seconds. After a bit of back-and-forth,...