Francis Reppert, Jr., 27, of Quakertown was charged in January following an investigation that began nearly a year ago.
Police say a student videotaped the now-former Palisades High School teacher viewing and zooming in on a photo of what appeared to be a student's legs.
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According to the Bucks County District Attorney's Office, the photos were taken of three female students from underneath Reppert's desk. Investigators also recovered photos of Reppert exposing himself.
He had been employed by the school district since August 2016.
Reppert's sentencing hearing will be held in the coming months. As part of his plea, Reppert he will be required to register through Megan's Law for at least 15 years.