The suspect wound up fleeing without getting the money, and no one was hurt, CBS Philly reported.
The incident happened at about 6:30 a.m. Monday at Saint Helena School.
According to investigators, the suspect approached a school door and pushed the buzzer stating he was making a delivery for the school.
After the suspect entered the building, he was met by a school employee on the second floor. Investigators say the would-be robber pulled out a handgun and demanded money.
According to CBS Philly, the employee led the suspect to a first-floor classroom where he was able to run away from the suspect and call police.
Then the suspect ran out of the school without getting what he came for.
Police say the school intruder is described as a white male or a light-skinned black male, 6'3" with a thin build. He was wearing a black jacket, blue jeans, a black baseball cap and a fake bushy beard.