Near the end of the high-school football season a few years ago, John Pizzi realized he had a problem. Because of season-ending injuries, the football team at Riverdale Country School, the New York private high school where he is the athletic director, did not have enough kids to finish the season. He canceled the team’s last game and then called Chris Nowinski, the CEO and co-founder of the Concussion Legacy Foundation, who has been talking for years about the need to better protect athletes