"He was doing what all kids do, playing, and then this happens," said Detective Sgt. Anthony Repalone, a spokesman for the Nassau County Police. "This was a senseless incident."
Authorities identified the victim as Michael Alguera, of Hempstead.
The teenager was playing handball with two friends on a court on the school's campus at about 3:30 p.m., about an hour after the end of classes, when they were confronted by as many as seven or nine young men.
The group robbed the boys of a cell phone and an MP3 player, then stabbed Alguera in the torso with a long knife. He was taken to Mercy Medical Center in Rockville Centre in grave condition and died at around 5 a.m., police said.
The two other robbery victims, ages 15 and 16, were unhurt.
A man who identified himself as Alguera's cousin, Alexander Flor, told Newsday he was a "good kid."
"He was just playing handball," he said.
Police on Saturday were still looking for the robbers, who fled on foot.
The handball courts are about 200 yards from the nearest high school building, alongside tennis courts in an area near a road, but secluded from the rest of the campus by a line of trees. Repalone said there appears to have been no other students in the area at the time of the attack, although investigators were still hunting for witnesses.
The detectives were probing a theory that the robbers approached the boys from a wooded area near the courts.
Their identities were unknown, but Repalone said investigators suspected from their descriptions that they were slightly older than their victims, possibly in their late teens or early 20s.
