The Norfolk County district attorney’s office has ruled the shooting of a Stoughton High School student in September was accidental. Seventeen-year-old David Wade, a defensive back on the football team, died after being shot in the chest by his 21-year-old brother in their Turnpike Street home on Sept. 29. The DA’s office said the pistol was lawfully registered and criminal charges were not warranted. Stoughton Police conducted the investigation with the State Police and the state medical examiner’s office. Norfolk County DA Michael Morrissey said his office also had the cooperation of the Wade family, including the brother involved in the incident.