Surveillance footage from the school shows about seven students playing basketball in the gym when one pulls out a handgun, Ramsey said. The gun goes off, hitting a boy and girl, both 15, and the shooter flees, he added.
The victims were taken to nearby Albert Einstein Medical Center, where they were in stable condition last night. Both had been shot in the arm.
One teen initially thought to be involved in the shooting was apprehended near his South Philadelphia home shortly after the incident, but was released when his involvement was deemed "not significant," police spokesman Lt. John Stanford said.
A teen connected to the shooting surrendered and remained in custody last night, Stanford said. Another was expected to turn himself in after his parents were contacted by police.
It wasn't clear who had fired the gun, Stanford said.
"We're not sure what happened, but the gun got into the school," Ramsey said. "Clearly, there was a security breakdown of some kind."
It wasn't clear last night whether the gun was fired deliberately, and police found neither the weapon nor any shell casings at the school, according to Ramsey.
"Situations like this shouldn't happen," he said. "Thankfully, there were no serious injuries, but any time someone is injured in a shooting, it's a tragedy."
Anxious parents waited outside the school last night, watching as students still in the building were escorted out by police one at a time. By 6:30 p.m., the school was cleared, police said.
One parent, Natasha Pressley, described the charter as a "good school" where security is normally tight - metal detectors scan every visitor at the main entrance, and guards also perform patdowns at checkpoints.
Still, Pressley said, security cannot stop every shooting.
"This could happen anywhere," she said. "I sent my kids here because of its reputation; it just goes to show that you're never completely safe."
Pressley's daughter made it out without incident, but a friend was forced to stay behind in the locked-down school, she said.
The daughter's friend told the Daily News last night that the lockdown occurred during dismissal, and that about half of the students were allowed to leave. The rest stayed in classrooms and conference rooms and were released in groups intermittently, she said.
The girl, whose identity is being withheld because of her age, said she wasn't sure what caused the shooting. She said that violent incidents have been common at the school in recent days.
One such incident Jan. 10 involved Robert Kane's son and daughter. Kane, who lives about a block from the school, said the shooting stunned him.
"As a parent, the whole thing is just scary," he said. "Someone now has to be in the hospital with their child, who was shot in school. Why? There's nothing a teen can do that's worth getting shot."
Kane said his son, a junior, and his daughter, a freshman, are currently suspended after a group of their peers picked a fight with them outside the school last week. He said they've told him that such behavior is becoming more common at the charter.
"It doesn't matter what kind of school it is; it could've been St. Joe's Prep," he said. "The students determine what happens inside those walls, and something seems wrong."
Terry Starks, outreach supervisor for Temple University's Philadelphia CeaseFire Program, said he rushed to the scene when he got the call that there was a shooting at his son's school.
"Your heart drops," he said. "These kids weren't shot on the playground or on the street, but inside their own school."
On Twitter: @Vellastrations