The former medical director at a venerable and exclusive prep school in Andover pleaded not guilty at his arraignment today on charges of possessing child pornography.

Richard Keller was a former medical director at Phillips Academy who also worked at Boston Children’s Hospital and taught at Harvard Medical School. He resigned last year and was arrested in September.

The 56-year-old faces up to 20 years in prison on charges of receiving child pornography and possessing child pornography. The charges carry a mandatory sentence of at least five years in prison.

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An indictment unsealed in federal court last week alleged that Keller possessed videos of minors, including boys from 7 to 16 years old, engaging in sexually explicit conduct.

His lawyer, Max Stern, would not comment on the allegations Tuesday, and would only reiterate that his client pleaded not guilty at his arraignment.

Keller, who appeared in court in sweatpants, is being held and has waived a right to a bail hearing.

Phillips Academy was founded in 1778 and has graduated legions of people who have gone on to be famous, including, for example, presidents George W. Bush and George H.W. Bush.