The King's School headmaster Tim Hawkes told the parents of a student who suffered an alleged indecent assault on a camp that the boy "bore some of the blame" for subsequent bullying, a royal commission has heard.

The school failed to make a formal report to police about the incident, despite a senior constable advising the deputy headmaster to do so, according to evidence to be presented to the Royal Commission into Institutional Responses to Child Sexual Abuse.

In his opening address, counsel assisting the commission David Lloyd told the hearing The King's School student, given the pseudonym CLC, was on a camp in April 2013 when another student allegedly ejaculated on his sleeping bag.

Mr Lloyd told the hearing that the teenage boy was bullied by other students and called "cum rag" and "cum dumpster". The inquiry heard a student changed the name of a Wi-Fi network at the school to "CLC's a cum rag".

The commission was told CLC did not report the alleged incident to the school because that would "make the situation worse".

CLC disclosed the alleged abuse to his father, EAE, in August 2013, the inquiry heard, and the school commenced an investigation.

Mr Lloyd told the inquiry that King's deputy headmaster Andrew Parry sought advice from a police officer, Senior Constable Robert Paterson, and was told in an email: "The incident/s should be reported to police so that the exact offence can be determined."

The email, read out to the commission, warned that failing to report a minor assault could result in criminal or civil action.

Evidence before the commission is that: "There was no contact between the school and the police in response to the email ... No report was made to police in response to that email, as recommended."

The commission heard police investigated The King's School in 2014 to determine whether anyone had concealed an offence but no charges were brought.

Mr Lloyd told the inquiry that EAE's testimony would be that Dr Hawkes said CLC was partly to blame his treatment by other students.

"EAE will give evidence that, during that meeting, Dr Hawkes said that CLC bore some of the blame and responsibility for the bullying because he did not report the camp incident when it occurred," Mr Lloyd said.

Dr Hawkes, Dr Parry, EAE and CLC have been called to give evidence to the two-week inquiry into alleged sexual abuse committed by students in both public and private schools.

The hearing will also examine a series of alleged sexual assaults at the Summer Hill school Trinity Grammar in 2000, involving students wielding dildos they made in their woodwork class and given nicknames such as "the anaconda", "the excalibur", "the dagger" and "the school house bat".

One victim's father wept as he told the commission: "We had no idea of the nature of the assault, that it was actually rape, that [my son] had been tied up and that it had happened to him at least 50 times."

The man, given the pseudonym EAA, was not contacted by the school for more than a month after allegations first arose.

"It felt like the school leadership was only interested in protecting the school," he told the inquiry.

The commission heard Trinity deputy headmaster Peter Green believed another alleged victim was exaggerating his story and did not believe he had been raped.

The hearing before Justice Peter McClellan continues.

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