The policeman, who was not identified, was a Tulane police officer and a reserve officer at the Second City Court, said New Orleans Police Superintendent Shaun Ferguson.
The officer was at a basketball game at George Washington Carver High School when an individual tried to enter, starting a fight with a team member, said Ferguson.
The policeman heard the disturbance and tried to remove the individual from school. As they approached the front door, the man drew his gun and fired a shot that hit the policeman in the chest.
The officer was taken to the University Medical Center New Orleans, where he later died.
The police have arrested someone they believe is responsible, Ferguson said, but no additional information has been provided about the suspect or a possible reason.