A 27-year-old social worker accused of sexually assaulting a 15-year-old inmate at the Charles H. Hickey Jr. School was acquitted of the charge yesterday by Baltimore County Circuit Judge Thomas J. Bollinger Sr. after her attorney argued that she had been raped by the boy.

Amy Bibighaus was charged with a third-degree sex offense - a felony charge prosecutors can bring against someone age 21 or older accused of having sex with a 14-year-old or 15-year-old. The charge carried a possible 10-year prison sentence.

"Justice was finally done," said Bibighaus' attorney, Domenic R. Iamele. "Not only was she victimized by the rape, she was victimized by the accusations made against her."

But Assistant State's Attorney John Cox says the office will not prosecute the teen-ager. "He didn't rape her," Cox said yesterday, declining to comment further on the acquittal.

Staff members at the juvenile detention center in Cub Hill called state police after one member of the Hickey staff said she saw Bibighaus and the teen-ager in a "consensual sexual position" during a meeting Feb. 12 arranged by the teen-ager's public defender.

But Iamele argued that his client had been attacked by the boy and that she had demanded to see a forensic nurse after the encounter.

Iamele also said the office had a malfunctioning lock, making it impossible for her to escape. A psychologist testified that Bibighaus suffered from "post-traumatic stress - rape trauma syndrome as the result of being brutalized," Iamele said.

Although Bibighaus has been cleared of the charges, her attorney said he wasn't sure whether she would be able to return to her job because it would mean meeting with other juvenile offenders.