It is the second such incident involving young Cranbrook boys and equally young girls in recent months and has again turned the spotlight on the early sexualisation of children and their constant bombardment, via the internet and the media, with sexually suggestive imagery.

The incident involving the year 8 boys is said to have taken place in the grounds of Bronte Public School after the girl sent a text message to one boy and arranged to meet him. The girl is alleged to have had consensual sex with one boy, but the two other boys present then allegedly engaged in acts, short of sex, with her as well. Accounts differ as to whether marijuana and alcohol were involved.

Cranbrook principal Nicholas Sampson said the boys had left the school.

“Reports of events that took place in the holidays came to the school's attention. When they came to us we worked with the families. As a result of those discussions, the boys are no longer part of the school community,” he said.

Another incident involving Cranbrook boys is alleged to have taken place in late September, at the end of term 3.

Two year 9 boys, aged 15, are alleged to have had sex with a very drunk year 8 girl at a party in Sydney's east while a third boy allegedly filmed it on his smartphone.

In that case, the school proposed referring the incident to the police.

However, the girl has since withdrawn her version of events and Fairfax Media understands that no charges have been laid.

The age of the children involved has horrified parents at the school and underscored the risks of the early sexualisation of children, the consumption of alcohol and drugs at parties and the lack of parental responsibility, especially at “gatherings”.

Unlike full-scale parties, “gatherings” usually involve between 10 to 30 adolescents and are often organised quickly. The parents, if they are home, are often blind-sided by the number of children who turn up and are unprepared for the level of drunkenness.

Paul Dillon, a drug and alcohol educator who provides courses to schools on dealing with adolescents, said the problem behaviour appeared to be occurring at younger ages.

“A lot of parents feel guilty about how little time they spend with their teenager. When they are with them they don't want a confrontation,” he said. “ If there's a phenomenon emerging, it's the collapse of boundary-setting."

On Tuesday night, after Fairfax Media's inquiries, Mr Sampson wrote to parents alluding to the events.

“Adolescence has always been a time when risks are taken as the world of adulthood is explored: it does not, however, license young people to suspend traditional morality or disregard either common sense or the law,” he said.

“As a boys' school, Cranbrook has a particular duty to encourage and enable its students to behave with courtesy and respect, particularly towards girls in their peer group."

The headmaster warned that the school intended to take “appropriate action” even when events happened outside school time. Where the matter involved possible criminal behaviour it would be referred to the police and the school would take disciplinary action, he said.

Adair Donaldson, a partner at Shine lawyers in Brisbane, who has developed training resources for the NRL on appropriate sexual behaviour, said there was enormous confusion out there about what was sexual assault and what was not.

He said it was important to explain to boys and men through interactive education where the line is drawn.

He warned there would otherwise be tragic consequences, not just for the victims, but for the young boys as well.

• Do you know more? investigations@smh.com.au