6ABC reported that police were investigating a report that the phone contained a video of a sexual nature. Flanagan said he could not confirm or deny whether the material on the phone was explicit or sexual in nature.
Radnor School District officials said state law prohibited them from commenting on the case because it involves minors.
Flanagan said the Delaware County district attorney had been notified. Police have also obtained a search warrant for material on the cellphone, which officers confiscated, he said.
He said that other search warrants could be forthcoming and that authorities were likely to press theft charges in connection with the stolen phone.
Police have interviewed four or five students, Flanagan said, and the school is launching a separate investigation.
School district officials declined to comment on whether students were suspended or expelled over the incident, and would not allow reporters into school areas to speak with students.
Police are focusing on one victim, Flanagan said, but depending on what they find on the phone, they could identify more than one.
"It's a great opportunity for people observing this to see how serious it can be if you send out [unauthorized] material to others," Flanagan said. "You could face criminal charges, even if you are underage."
Contact Aubrey Whelan at 610-292-8112, firstname.lastname@example.org, or follow on Twitter at @aubreyjwhelan.