The incident occurred inside the gymnasium at the Philadelphia Learning Academy - South on the 4300 block of Westminster Avenue, Fox29 reports citing City Police Sgt. Eric Gripp.
Police rushed the teen to the hospital where he was reportedly in stable condition.
Nearly 170 people were evacuated from the building.
It was unclear how the student go the gun inside as students do go through metal detectors.
The weapon was not immediately located.
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