Police say at approximately 10:45 a.m. Friday, 21-year-old Kelley Jones was being screened at the lobby at South Philadelphia High School.
Jones, of the 2500 block of South 69th Street, was accompanied by an infant child and another female relative, police said.
At that time, school police officers asked that their coats and belongings, including a diaper bag, be placed on the scanning conveyer belt.
The three visitors then continued on the screening process.
While the officers scanned the belongings of the woman, they noticed a firearm.
The school officers further investigated and recovered a loaded .40 caliber Smith & Wesson semi-automatic handgun.
"When the weapon was examined it was discovered that the firearm's serial number had been obliterated," Philadelphia Police Captain Laurence Nodiff said.
Police say Jones did not have a permit for the weapon.
"She is claiming she was not aware that the loaded [gun] was in the diaper bag," Nodiff said.
Jones was placed into police custody and transported to South Detectives.
On Friday, Jones was formally charged with possessing a gun without a license and bringing a weapon onto school property. Her bail was set at $200,000.
Jones tells police she was at the school to register for a night course.
While Nodiff said Jones is no stranger to authorities, he said he does not have any information that she came to the school other than to sign up for the evening class.
"The security measures worked well and the school police officers did their job and stopped the threat right at the front door," Nodiff said.
There were no injuries during the incident.