A teacher at an Elk River charter school has been charged with third-degree criminal sexual conduct after allegedly admitting to having a sexual relationship with a 17-year-old student.

Matthew C. Ellsworth, 36, of St. Paul, told police that he had sex with the student six to eight times during the summer, according to a Ramsey County District Court complaint filed against him last week.

The charges say he taught the student during her junior year at Spectrum High School and was her mentor for her senior project, which required them to meet during the summer.

The student turned 18 on Aug. 30. According to the charges, police spoke with her outside Ellsworth's apartment building in the 1800 block of Randolph Avenue in early August after her sister, who knew of the relationship, said that she was there.

The student told police that she and Ellsworth kissed for the first time in June and began to have sex a few weeks later. They knew the activity was illegal, she said, and discussed waiting until she was 18. Ellsworth confirmed those discussions, the charges state.

He has been charged with third-degree criminal sexual conduct because she was 17 and because he was more than two years older than she and in a position of authority.

On Monday, school administrator Vanessta Spark said that Ellsworth was employed at the school from August 2010 to August 2011, "when he was terminated for reasons unrelated to the criminal charges." Because of privacy rules, she said, the school could make no additional comment.

ANTHONY LONETREE