The woman, who cannot be named for legal reasons, was a 17-year-old schoolgirl when Ben Fenner began sending her sexualised messages via Whatsapp.

"All the messages on here are encrypted ... great for secrets," he messaged her, according to the agreed facts.

Fenner has pleaded guilty to seven counts of sexual intercourse with a person under 18 and under his care.

He asked Judge Robert Weber to take into account a further eight matters of sexual intercourse with his former student.

On Thursday in the District Court the woman read out her victim impact statement saying it's taken a long time to realise she was manipulated and groomed by her teacher.

"I thought this was a normal thing ... a normal relationship," she said.

"He knew the rules and the code of conduct ... he used me ... I was a vulnerable 17-year-old, you were 33."

At the time of the abuse she was suffering from depression and anxiety and had panic attacks after the loss of a family member.

"I feel you have taken so much away from me ... my friends and high school experience," she said.

She said she was proud she reported the "perverted mess" to the police so that Fenner could not use his position of power to abuse anyone again.

After finishing her statement she stared straight at Fenner who had his head bowed.

Giving evidence, Fenner apologised for causing her grief and pain, and ruining the time leading up to her Higher School Certificate.

When asked if he knew what he was doing was wrong, a serious beach of trust and an abuse of the teacher-student relationship, he continually answered "yes".

"You noticed there was some marks on her neck you thought were hickeys ... After that you asked what she liked sexually," crown prosecutor Roger Kimbell said.

"That was completely inappropriate wasn't it?"

Fenner was born in England but moved to Sydney to start a new life after the breakdown of his marriage in 2011.

He described the philosophy at the eastern suburbs school where he taught biology as significantly different to other schools he had worked at.

Teachers were encouraged to give students their personal phone numbers and to be available for contact out of regular school hours.

His text messages to his victim turned sexual before organising to meet up outside school grounds over a three-month period.

In one instance she thought they were going to the beach for a school assignment, but instead he had sexual intercourse with her.

Defence lawyer Phillip Gibson said Fenner felt unable to extricate himself from the situation, fearing the depression and anxiety he experienced in past relationship breakdowns.

While Fenner was not suffering a severe mental illness at the time, leading up to the period of offending he had regressed to an "low point emotionally".

The Crown said there has been a gross power imbalance between a child and adult.

"It's not a case of a relationship between two consenting adults," Mr Kimbell said.

"He's an intelligent professional man in his 30s almost twice her age."

Judge Weber will hand down his sentence on October 9.

Australian Associated Press