Four suspects, ranging in age from 15 to 19, were arrested in a sweep of the neighborhood near John McDonogh High School. Police Chief Eddie Compass said he did not know if the suspects attended the school.
Students said the shooting was apparently gang related and may have been retaliation for a previous fight.
"They started shooting and I started running," said ninth-grader Garick Jacob, who was in the gym when the shooting began. "I was really scared."
The gunman managed to slip out of the gym and the suspects were arrested about three blocks away. Two were in a getaway vehicle and two others were in a nearby house in the Mid-City neighborhood, about a mile north of the French Quarter.
It was not immediately clear how the gun got through metal detectors and guards at the school. Students and school security officers said there was a hole in the fence near the gym.
School board member Elliot Willard said students told him that the boy was the target and the girls were accidental victims.
Charity hospital spokesman Jerry Romig said a 15-year-old girl had surgery because a bullet went through both her legs. The other injuries were not considered life-threatening.
Parents rushed to the school only to find their way blocked by the police lines. Darlene Claiborn said her daughter called her from inside a classroom on a cell phone.
"How can this happen in a school?" she demanded. "They have guards in there. They're supposed to have security."
Anthony Amato, school system superintendent who has been on the job only since February, tried to calm parents by saying that officers had swept through the school and that the students were safe. He was repeatedly interrupted by cries of, "Where was the guard?"
In another, unrelated campus shooting Monday, a gun went off in a Shreveport, Louisiana, school — apparently accidentally — while one student was showing it to another, hitting the second boy in the stomach, police and school officials said. The student who brought the gun ran away; police were looking for him. The wounded student was in serious condition.