The Rose Bay Secondary College student was found guilty at Parramatta Children's Court on Monday, charged with producing child abuse material and disseminating child abuse material.

He was sentenced to 18 months probation and ordered not to approach, contact or communicate with the teenage girl during that time, by way of apprehended violence order. He will not have his conviction recorded.

The boy was among teenagers who attended a party in Bellevue Hill earlier this year, where it is alleged another 15-year-old boy sexually assaulted a teenage girl who was intoxicated.

The latter boy was a student at the exclusive Cranbrook School, and was charged with sexual assault three weeks after the incident, when footage allegedly circulated between scores of students in a private Facebook messenger group.

The incident was reported to police in an anonymous phone call informing them of the 13-second video.

On Monday the court heard that the young person responsible for filming the alleged assault "did not think about the consequences or negative impact it would cause".

Parramatta Children's Court magistrate Estelle Hawdon said the 15-year-old had "some difficulty understanding the legal concept of consent", referring to an assessment detailed in a Juvenile Justice Service report.

"I have to find it was an offence characterised by immaturity ... impulsiveness and lack of consideration," she said.

The court heard the boy had sent a text message saying that he filmed the incident "for revenge" on the 15-year-old who allegedly assaulted the teenage girl.

In another group chat conversation detailed in court documents, the boy said police would be unable to prove he had filmed the act, writing "How r they gonna see if it was me behind the camera ...Someone coulda taken my phone [sic]."

Ms Hawdon found the fact the material had not been produced for "sexual gratification," but rather "for revenge" was "even more insidious ... I find that extremely difficult to come to terms with."

"His total disregard of [the victim] as a person ... because he filmed for revenge, is something that is not to his credit and I don't find it ameliorates his actions in any way," she said.

However Ms Hawdon also noted the boy had "greater insight and feels bad for the victim and her family".

As part of its submission, the prosecution outlined two aspects of criminality; the production of the video and the dissemination of it.

"By sending to just one other person the young person could not control who saw it or if it was further disseminated," the prosecution argued, adding that many people under the age of 16 years saw the video as a result.

"The young person disseminated the film knowing it would be shown to others ... he must have known there was a real risk of it being disseminated further ... [causing] significant humiliation and distress to the complainant."

The boy's defence lawyer said the case spoke to the "growing trend of young people, unfortunately, to film everything that occurs, particularly when they are intoxicated".

"These are matters he will [deal] with for the rest of his life ... There is no doubt he certainly has had to deal with the seriousness of his conduct."