A former Sydney high school teacher who pleaded guilty to sexually assaulting a 14-year-old boy has broken down in court while detailing her remorse over the "severe, lifelong" impacts of her actions.

Monica Elizabeth Young, 24, pleaded guilty in April to three charges of aggravated sexual intercourse with a boy aged 14 in June and July 2020.

The charges were aggravated due to the child being under her authority at the time, a court heard in April.

Young sobbed as she read aloud a letter of remorse to the court and District Court Judge Kate Traill at her sentencing hearing on Thursday.

"I am extremely embarrassed and ashamed that I have to write this letter to Your Honour; I am unable to fully express how sorry I am for my actions," she told Downing Centre District Court.

"I cannot imagine how [the boy] and his family felt … it will be something they carry with them for the rest of their lives and it's not something I can take back or change.

"My intentions were never to hurt him. [He] trusted me and I abused that trust which may have severe impacts on him in the future."

Her high-profile barrister Margaret Cunneen SC told the court Young, who was engaged at the time of the offences, "was in a relationship of several years … but it was going very badly".

Cunneen said the young teacher started her fulltime teaching career in March 2020 and "didn't get into teaching to access young boys, far from it".

"She was really just out of school [university] herself, if I may make that submission, your honour.

"Ms Young appreciates she can never again be a teacher."

The court heard Young pursued the boy on social media and the pair exchanged nude photographs.

She later instigated sexual acts with him, including during a movie screening with other students nearby.

She was caught on CCTV driving to meet her victim in a park after messaging him words to the effect of "it's dangerous if we get caught, but if you do it, it'll be worth it".

But his family saw them together in the car and police were alerted.

Sydney teacher Monica Young wept in court. Photo / Facebook

Prosecutor Alexander Terracini urged Judge Traill to consider as she decides on a sentence that Young sexually assaulted a student within months of starting fulltime teaching.

"While it may be accepted Ms Young didn't go into teaching to have access to children the fact is within months … she was abusing one of them," he told the court.

"That cannot simply be ignored."

Under questioning from Terracini, Young accepted she initiated the physical contact with the boy on each of three occasions across a two-week period.

Young was first arrested at her home in Greenacre in July but was released on bail from Silverwater Prison under strict conditions in August.

On Thursday, she described the five weeks in prison as, "confronting", while also telling the court she has had no access to internet while on bail for the past 10 months and has chosen not to have a mobile phone.

"It has been quite surreal to be on bail as I never thought I would be one of those people," she said.

Young will be sentenced on July 8.

- Evin Pries, news.com.au