A "handsome and charismatic" orchestra conductor and composer used his charm and highly regarded reputation to seduce and abuse three teenage students almost three decades ago, prosecutors have told an Adelaide court.

Key points: Timothy Sexton, 59, is accused of 17 child sex offences against three singing students

Timothy Sexton, 59, is accused of 17 child sex offences against three singing students The crimes were allegedly committed from 1989 until 1993 when he was a music teacher

The crimes were allegedly committed from 1989 until 1993 when he was a music teacher The accused later became chief executive of the State Opera of SA

The 59-year-old former State Opera of South Australia chief executive is accused of 17 sexual crimes against three girls across seven Adelaide suburbs between 1989 and 1993.

Charges include two counts of maintaining an unlawful sexual relationship with a child, seven counts of indecent assault and a further eight counts of unlawful sexual intercourse with a child.

Opening the trial in the District Court, prosecutor Carmen Matteo told the jury that his three alleged victims were "beguiled" by him.

She said the music teacher was "talented, handsome and charismatic".

"Mr Sexton was highly regarded and he was liked," she said.

"This was an ordinary state of affairs which gave him the opportunity and freedom to sexually offend against these girls."

Ms Matteo said Mr Sexton was already an accomplished composer but would work part time in schools across Adelaide to teach singing.

"They were children and the accused was their teacher," she said.

Students were 'carefully and effectively seduced'

Ms Matteo told the jury that each of the alleged victims was "flirtatious" with Mr Sexton, and that he reciprocated.

"He's in his late 20s, married, legitimately providing [alleged victims] with singing lessons and his conduct in private moments would look to a 16-year-old as a grown-up love affair," she said.

"Look, touch, comments, little gifts — all very powerful things in the world of a teenage girl.

"Each girl, on the prosecution case, describes being gradually, carefully and effectively seduced by their singing teacher."

She said the girls were "willing participants" but each was underage at the time of the alleged crimes, and that willingness did not "exonerate" an adult from sexual activity with a child.

The trial is occurring in Adelaide's District Court. ( ABC News: Gary Rivett )

Ms Matteo said Mr Sexton kissed one alleged victim — aged 15 at the time — on the cheek after a singing lesson in 1989.

"He said to her, 'if I give you the sort of kiss I want to, you'd be shocked or surprised' … her response was, 'maybe you should'," Ms Matteo told the jury.

She said the accused then allegedly kissed the girl on the mouth and, on a later occasion, allegedly had sexual intercourse with the girl.

The court was told that the three alleged victims were not known to each other, but had a "shared experience" through their dealings with Mr Sexton.

Mr Sexton pleaded not guilty in the Adelaide Magistrates Court in 2018 to offences including maintaining an unlawful sexual relationship with a minor and indecent assault.

He was the artistic director and chief executive of the State Opera from 2011 until his resignation in May 2017.

The trial, before Judge Geraldine Davison and the jury, continues.