On Valentine's Day 2018, an expelled student walked into Marjory Stoneman Douglas High School in Parkland, Fla., and opened fire with an AR-15 semiautomatic rifle. Fourteen teenagers and three coaches died. Parkland students and parents decided school shootings must end. They channeled grief into action, launching a campaign to halt school violence. America's gun-rights activists responded in a ringing national debate. Watch our five-part series here.