SHE was the "young, attractive and popular" coach of the high school girls' football team.

He was the talented captain of the boys' team, a student at her school and 17 years old.

Now, eight years later, the Sydney PE teacher is on trial charged with engaging in a sexual relationship with the boy over a four-week period in 2005.

The 30-year-old woman, who can't be named for legal reasons, has pleaded not guilty to three counts of having sexual intercourse with the teenager who was under her care.

The District Court trial on Monday heard the Catholic school teacher, then aged 22, did not teach the boy, but got to know him after they both attended a weekend outing at Luna Park to celebrate the girls' team reaching the finals.

"(She said) I was one of the most mature guys at school and it felt more like a friendship," the former student, now aged 25, told the court.

He described how over the following three Wednesday afternoons, the teacher picked him up on a side street near his home and drove him to a secluded area near a soccer stadium where they engaged in sexual acts.

The jury was told the sexual intercourse charges were based on the allegation the boy digitally penetrated the teacher on those occasions.

The man said he couldn't remember exactly what time of year it was, other than towards the end of Year 12.

He told the court that after the teacher began talking about wanting to have children one day he got scared and suggested they should just be friends.

"She said no, either we're in a relationship or no friendship," he said.

After the meetings ended, there was no more contact between them for the rest of the school year, he said.

The student admitted he lied when the alleged relationship was investigated by the Catholic Education Officer in 2008.

He told the court the teacher rang him in tears about it.

"I said don't worry about it, I'll deny everything," he told the court.

Charges were eventually laid against the woman last year following a police investigation.

Defence barrister Phillip Boulten, SC, told the court the woman never had a sexual relationship with the student whose claims were "empty and false".

"She was the subject of a lot of attention at school because she was young, attractive and popular," Mr Boulten said.

He put to the former student that he and his friends treated the teacher, who had just graduated from university, as a "sexual object".

"You were making a big name for yourself at school by making out that you had got with (her)," Mr Boulten said.

The former student denied this.

Mr Boulten said there was no forensic evidence, CCTV footage or phone records to back up the Crown's case.

"There will be two witnesses in this case who will shed light on whether it did or it didn't happen," Mr Boulten said.

They would be the teacher and the alleged victim, he added.

The trial continues before Judge Leonie Flannery.