The incident happened at 6:30 a.m. Monday at the Saint Helena School at 6101 North 5th Street.
According to investigators, the man hit the buzzer and said he was there to make a delivery.
He entered the building and was met by an employee on the second floor.
That's when the man pulled out a gun and demanded money, police said.
The worker led the would-be robber to a first floor classroom and said, "It's in there." That's when the administrator was able run down the hall and call police.
The suspect fled in an unknown direction without getting any money.
Police say the suspect is described as either a white or light-skinned black male standing 6'3" tall with a thin build. He was wearing a blue jacket, blue jeans and a black baseball cap.
Police also say the man was wearing a fake bushy beard.
Anyone with information is asked to contact Philadelphia Police at 215-686-TIPS (8477), text a tip to PPD TIP (773847) or through phillypolice.com.