Updated Jan. 18, 2014, 3:53 p.m.
The boy’s identity is not being released because of his age, but he has been charged with aggravated assault. The first boy to turn himself in Friday has not yet been charged.
Both victims, a male and a female at Delaware Valley Charter School, were shot in the arm in the school gym, the Associated Press reports. They were taken to a nearby hospital, where the girl has been released and the boy is listed as in stable condition, police say.
Police Commissioner Charles Ramsey said at a news conference that the weapon has not been found, and it remains unclear if it was discharged intentionally or by accident.
This story has been updated with more details on the shooting.