A 75-year-old former teacher at one of Perth's most prestigious girls' schools is on trial in the District Court accused of sexually abusing five teenage students between 30 and 40 years ago.

Jay Riordan Walsh is facing 23 child sex charges, which allege that in the 1980s, when he was a drama teacher at Penrhos College in Como, he abused the girls at various locations including a motel room, on school grounds and in car parks.

State prosecutor Kim Jennings said two of the women lost their virginity to Mr Walsh, who at the time was in his 30s and 40s.

Ms Jennings described Mr Walsh as "well-respected" and "a great and enthusiastic drama teacher" but she alleged that with these five women, "he crossed the line".

She told the court that while he enjoyed good a reputation and had the trust of parents and the school community, that may have been one of the reasons why he was allegedly able to abuse the students without anyone knowing.

A power imbalance

The court heard some of the students had a crush on Mr Walsh and considered that they were in "a romantic relationship with him".

However, she said the law draws a line and says that sex between a teacher and their student is not allowed because of the power imbalance between the two.

The allegations first came to light in 2018, after the then Penrhos principal sent an email to the school community in response to the Royal Commission into Institutional Child Sexual Abuse.

Ms Jennings said some of the women contacted the school but there was no formal report to police until 2020 when one of the former students made a complaint.

Her allegations were put to Mr Walsh in a videotaped interview with police, but Ms Jennings said he denied any sexual relationship or sexual conduct with the former student when she was a teenager.

Mr Walsh denies all the allegations against him.