Police say the game was being played by kids ranging from 12 to 16 years old.
At one point, an argument broke out between two 16-year-olds over the ball.
One of the teens left the court, police say, and went home to get a gun.
He returned to the courts on his bike and shot the other 16-year-old.
The victim was taken to the Hospital of the University of Pennsylvania in stable condition.
Police are now searching for the shooter.
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