A teacher's aide was arrested Friday at the Lafayette elementary school where she works on suspicion of sexually assaulting a sleeping woman at a home near Walnut Creek last year.

The charges were unrelated to Springhill Elementary, where Stephanie Elliott works with special education students, and did not involve any of the students, police and prosecutors said.

"This had nothing to do with the school," said Nancy Georgiou, head of the Contra Costa District Attorney's sex crime unit.

Elliott, a 39-year-old Brentwood resident, was booked into County Jail in Martinez on felony charges of oral copulation and attempted sexual penetration. The charges allege the acts were committed upon an unconscious person.

Elliott was released from custody after making $100,000 bail, said Contra Costa Sheriff's spokesman Jimmy Lee. Elliot was arraigned Monday in a Contra Costa County courtroom. She is scheduled to return to court to enter a plea Dec. 22.

Georgiou said the accuser is an adult, and the incident happened in July 2010 in a home just outside Walnut Creek.

The allegations were referred to the district attorney's office last year, but there was insufficient evidence at that time to prosecute, Georgiou said. Superintendent Fred Brill said Elliott was arrested in the school parking lot, and he did not believe many students witnessed the incident.

Elliott has been placed on administrative leave, and parents were notified of the arrest via email, Brill said. Since the arrest did not involve students, no parent meeting was planned, he added.

"Obviously, this is something where we have concerns, and what we want to do in situations like this is make sure we're continuing to focus on our priority, which is the education and the safety of our kids," he said. "And that's what we continue to focus on."

Brill said the district would provide other aides to the students with whom Elliott worked.

"We have a number of aides in the district," he said. "We'll make sure there's appropriate support for our children."

Staff writer Matt O'Brien contributed to this report.