A children’s magistrate who sentenced a former private schoolboy over the rape of a 15-year-old girl at an eastern suburbs party has urged the teen to not let the crime “hold you back”.

The boy, who was 15 at the time of the sexual assault, has been sentenced to 20 months’ probation over the 2017 incident, which was filmed by a friend and widely distributed.

The boy pleaded guilty earlier this year to aggravated sexual assault and to disseminating child abuse material.

He was also sentenced to 18 months’ probation for distributing the disturbing video, an order that will be served at the same time as the larger probation period.

After delivering her sentence, magistrate Sue Duncombe told the boy that the sexual assault would not determine his future.

“Don’t make this hold you back," she said.

“This is a very big, but only one mistake. Sorry, two mistakes,” Ms Duncombe told the teen, who hugged his mother in court following the sentence.

Ms Duncombe also criticised the hosts of the party, saying the lack of adequate supervision “possibly contributed to what happened”.

The offender has already suffered serious loss and detriment as a result of committing the offence.

Sue Duncombe, magistrate

The teenage girl was unconscious during the incident and was only made aware of it after being told by friends that a video had been circulated.

Teachers at the boy’s school reportedly called police after learning mobile phone footage of the attack had been shared among dozens of teenagers via a private Facebook Messenger group.

Another youth has also been handed an 18-month probation order for filming the attack and distributing the 13-second footage.

The teen who had sexually assaulted the girl claimed he was “in shock” when he sent the footage, but Ms Duncombe rejected this.

But she said the boy had been subjected to “significant extra-curial punishment” in the wake of the scandal, which had been widely publicised in the media.

“This young person has had to change schools, change residence,” Ms Duncombe said, adding that he had also received online abuse.

She said that in leaving the area, the boy also had to give up his participation in a number of sports teams.

“The offender has already suffered serious loss and detriment as a result of committing the offence,” she said.

The teen’s lawyer had asked Ms Duncombe dismiss the charges, which she refused to do.

No conviction will be recorded given he was under 16 at the time of the sexual assault.

But she said a number of factors, including the boy having no criminal history, his early guilty plea, and his apology to the girl, would mitigate his sentence.

“I accept the young person has demonstrated remorse,” she said.

“He understands the seriousness of these charges and the effect on the victim.

“He clearly has good prospects of rehabilitation.”

The court heard the teen aspired to join the navy, which Ms Duncombe said demonstrated that the teenager wanted to “serve the community”.

A $10,000 surety previously put up to ensure the boy’s return from overseas, where his father works, was returned.

The boy is to report to NSW Juvenile Justice within a week.