Outside the Roger Clap Innovation School in DorchesterFriday afternoon, parents waited to pick up their children with warm embraces and anxious hearts.

As excited students raced out of the school’s double green doors, their parents faced the uncertain prospect of how to discuss the school shooting in Newtown, Conn. Experts offered advice and schools scrambled to provide guidance. They all recognized the challenge ahead: How could a parent explain to a child an event so horrific that adults had trouble comprehending it?