The incident happened at Mastbaum Area Vocational Technical School, located at 3116 Frankford Avenue.
Investigators say it was around 3:00 p.m. when school was letting out when two students and a third individual wearing a mask and armed with a gun robbed a student on the third floor of the school.
Stolen was the student's cell phone and a PSP video game system. The victim was uninjured in the incident.
The lockdown at the school was lifted around 4:00 p.m.
It is believed the unidentified armed suspect gained access to the school from outside the building. There are security guards posted at the front of the school, but the suspect somehow got in during dismissal and committed the robbery.
Police say the robbery was caught on surveillance and that they are currently working to obtain the footage.