A former Montgomery County music teacher pleaded guilty yesterday to one count of child sexual abuse, admitting that she had a three-year affair with a male Rockville High School student 10 years ago.

Phoebe J. Rockwell, a Montgomery choral teacher and theater adviser for 25 years, faces up to 15 years in prison when she is sentenced Thursday, prosecutors said. However, Maryland sentencing guidelines call for probation to two years in prison. Rockwell last taught chorus at Wootton High School. She was placed on administrative leave upon her arrest and then retired, her attorney said.

Montgomery police said Rockwell, 47, met the student when he was a 15-year-old sophomore in a school play she was supervising. She engaged in sexual activity with him at her Gaithersburg home, in her chorus office and in her car, ending the affair after the youth went to college, police said.

The student, now 26, told someone about the relationship late last year, and that person was required by Maryland law to tell police, said Assistant State's Attorney Kristen Bender. Those legally required to report possible sexual abuse include teachers, doctors and therapists. Bender would not say who reported the abuse in this case.

"We'll be asking for a sentence that reflects the breach of trust that a teacher commits when she has sex with one of her students," she said. "He was a very shy kid, and she paid him a lot of attention and really built up his ego. . . . However, he also had this huge secret he couldn't share with anybody that really deprived him of a normal high school experience."

Prosecutors charged Rockwell with the one count of child abuse. Rockwell, who also goes by her former married name of Krajewski, said little during yesterday's hearing, except to answer yes to Circuit Court Judge Vincent E. Ferretti Jr.'s routine questions about whether her plea was voluntary. She did not return a phone message left later at her home.

Her attorney, Tom Morrow, said the former teacher pleaded guilty because she "wanted to avoid putting everyone through a trial."

"At this point, having entered the plea, it would serve no purpose to regurgitate her version of the events," Morrow said.

The former student told police that he met Rockwell in 1987 during auditions for a school play. He confided in her and eventually developed "romantic feelings" for her, police wrote in charging documents.

The two eventually engaged in oral sex and intercourse, police said.

The former student attended but did not speak at yesterday's hearing and did not return a call to his home later. Bender said she does not want Rockwell to go to prison and will ask the judge for leniency in her sentencing.

"He thinks she used bad judgment and took advantage of her position of trust," Bender said.