Policed said one other student was in custody and another was being sought. It wasn't clear whether either was suspected of being the shooter. A third student was questioned and released.
The gun used in the shooting had not been recovered.
Lt. John Stanford told reporters investigators were not sure whether the shooting, which took place in the school's gymnasium shortly before 3:30 p.m. EST, was intentional or accidental but they had video surveillance footage of the incident and were looking for the shooter. There were seven students in the gym when the gunfire broke out.
"Come turn yourself in now," Stanford advised the shooter during a news briefing. "Get it over with."
KYW-TV, Philadelphia, reported the victims were taken to a nearby medical center for treatment. The nature of their wounds was unclear
Stanford said the victims -- a girl and a boy, both 15 -- were in stable condition.
The school was placed on lockdown until 6 p.m. and students were released for the day.