It happened near PS 119 just after 7 p.m. on Saturday.
The suspect then walked up East 39th Street towards Avenue K and onto the playground at PS 119 where a 15-year-old boy was shot in the torso.
The 15-year-old was pronounced dead at the hospital.
The 13-year-old is in serious condition at Kings County Hospital.
The gunman fled the area on foot.
So far there are no arrests.