Retha Moody walked across the asphalt to the beat of a clanging flagpole, past the barbed-wire fence, toward the red brick building, balancing a long white box in her arms. She led the handful of relatives she had assembled through the metal doors and signed in at a glass booth. After securing their personal belongings in lockers, the group paraded through metal detectors before being led up to the second floor, past several doors that locked behind them. The family had come to the Department of Youth Services facility in Dorchester to celebrate the 15th birthday of Raeshawn Moody, who...