The King's School in Sydney demonstrated a "catastrophic failure" in its handling of a child-sexual assault allegation, a submission to a royal commission states.

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 King's School's then headmaster Timothy Hawkes outside the royal commission after giving evidence. Credit:Daniel Munoz

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.