The submission by David Lloyd, counsel assisting the Royal Commission into Institutional Responses to Child Sexual Abuse, stated that the school was advised to report the allegation to police but failed to do so.

The claim relates to a 2013 incident at a cadet camp in which one student was alleged to have masturbated on another student's sleeping bag.

In a public hearing into harmful sexual behaviour in schools, the royal commission was told the Parramatta school sought advice from a police officer who wrote an email saying a formal report should be made.

The commission heard The King's School's then headmaster Timothy Hawkes did not make a report to police.

"The failure to report the ... camp incident to police, contrary to written advice from the police that the matter should be reported, was a catastrophic failure by King's," the submission read.

Counsel assisting also stated: "As of 2013, a bullying culture existed at King's both inside the boarding houses and in the school more generally."

In its submission, The King's School rejected the claim of "catastrophic failure".

"It is respectfully submitted that rhetorical flourishes in the course of cross-examination should not be used as a basis for factual findings," the submission read.

"The expression 'catastrophic' in this context is misplaced and exaggerated. This finding [sic] should be rejected in its entirety."

The King's School submitted that Dr Hawkes could not remember receiving the email in which the officer advised making a police report about the alleged assault, nor could he recall reading it.

The school also submitted that it was not open to the royal commission to make a finding about an alleged bullying culture at the school.

Harmful sexual behaviour in schools was the subject of a public hearing held in Sydney in 2016.

The commission also heard evidence about a series of sexual assault allegations involving Trinity Grammar School students in 2000.

Counsel assisting submitted that Trinity Grammar headmaster Milton Cujes was given a report alleging: "There had been simulated rape or that a dildo had been shoved up boys' bottoms on multiple occasions. The only response by Mr Cujes to the information he received from (deputy headmaster Peter Green) was to suspend the boys involved for the weekend and to await an informal investigation."

The submission found that Mr Cujes "should have done more to investigate the serious allegations ... with a view to reporting those allegations to the police. Mr Cujes did not take steps to investigate the allegations ... or report the matter to the police."

Trinity Grammar School and Mr Cujes submitted that more could have been done at the time of the alleged incident but practices had improved over the past 17 years.

"The school accepts that it, through its staff, did not act perfectly in response to the events that occurred on 11 August 2000," the submission read.

"The head master has acknowledged that the school could have and should have done more.

"The school submits that it has, since the events of 2000, taken up the opportunity presented by those events to learn and act on how best to deal with this complex issue."

The royal commission will hand down its final report into harmful sexual behaviour in schools at a later date.