Investigators say Donte Walker, 18, of the 5600 block of North Sydenham Street, appears to have provided the gun that was used inside the gymnasium of the Delaware Valley Charter School.
The exchange was apparently made inside the school, police say. The gun was allegedly fired by Raisheem Rochwell, 17, of the 7400 block of Rugby Street, just moments later.
Walker is a 2013 graduate of the school, police said, and did not have to pass through metal detectors because he was considered a 'guest.'
According to investigators, Rochwell feared he was going to be the target of an assault and may have sought out a gun for that reason.
Police believe Walker gave Rochwell the gun and allege there was an exchange of cash. However, investigators are now backing off a statement made earlier Monday that a third person gave Walker cash in an apparent sale.
That exchange happened just before the gun was fired, police say. A bullet struck an 18-year-old female student in the arm, passed through and hit a 17-year-old male student in the shoulder.
Both students have since been treated and released from the hospital.
Donte Walker has been charged with Violation of Uniform Firearms Act, Conspiracy and related offenses.
Raisheem Rochwell surrendered to police on Saturday, was charged as an adult with Aggravated Assault, Violation of Uniform Firearms Act, Recklessly Endangering Another Person, and weapon on school property, and other related charges.
Rochwell's attorney, Amato Sanita, declined to comment on the specific circumstances of the shooting on Saturday, but said Rochwell is a good kid with no prior criminal record.
"I'm saying it's not intentional and we're not admitting any fault on his part," Sanita said.
His client remained in custody on $500,000 bond, with a preliminary hearing scheduled for Feb. 6.
A 15-year-old who was taken into custody in South Philadelphia shortly after the incident was later cleared of any wrongdoing and released.
The shooting remains under investigation.
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.