BOSTON – A doctor who worked at a prestigious boarding school is finalizing a plea deal with federal prosecutors in a child pornography case.

Richard Keller, of Andover, pleaded not guilty last year to possessing and receiving child pornography. Authorities said he had some of it shipped to the student health center at the elite Phillips Academy in Andover, where he was the medical director for 19 years until stepping down in 2011.

Prosecutors and Keller’s attorney filed a report in court this week saying they have had plea negotiations over the last several months and have reached an agreement in principle on a plea deal. They asked to schedule a change-of-plea hearing for next month to give them time to complete details.

Keller’s lawyer, Max Stern, and a spokeswoman for U.S. Attorney Carmen Ortiz declined to comment. Neither would say whether Keller will plead guilty to all three charges or whether the two sides will make a joint sentencing recommendation.

Keller, 57, faces a maximum of 20 years in prison, including a five-year mandatory minimum on the charge of receiving child pornography. He is charged with having pictures and video of boys from ages 7 to 16 engaged in sexually explicit conduct.

Keller resigned from Phillips Academy after school officials said they would not renew his contract.

John Palfrey, the head of school at Phillips Academy, said in an email to students and parents after Keller’s arrest that the decision to not renew Keller’s contract was based on professional misconduct unrelated to the federal case. Palfrey said Keller had been reprimanded in 1999 for using a school computer to access adult pornography and had shown an inappropriate cartoon to students in 2002.

Keller also worked as a pediatric endocrinologist at Children’s Hospital in Boston and taught at Harvard Medical School. He voluntarily surrendered his state medical license.