The shooting happened at the Delaware Valley Charter School in north Philadelphia around 3 p.m., they said.
Surveillance video shows a group of about seven students hanging around when the shooting happened, police Commissioner Charles Ramsey said. It was unclear if the shooting was accidental or intentional, but it "doesn't look like an active shooter type of situation," he said.
A male student and a female student were shot, each in an arm, but it was unclear whether one or two shots were fired, Ramsey said.
The wounded students were taken to a hospital near the school, where they were listed in stable condition, police said.
The shooter lives quite far from the school and may have driven or taken a bus toward his home after the shooting, Ramsey said. No gun was recovered, he said.
A spokesman for the school district didn't immediately respond to a call seeking comment.
It's the second U.S. school shooting this week. Authorities said a seventh-grader opened fire Tuesday in a New Mexico middle school gym, wounding two classmates.