The deputy headmaster of Trinity Grammar School was told a student's underpants had been torn off and older boys had attempted to rape him but did not believe the allegation warranted a report to police, a royal commission has heard.

Trinity Grammar deputy headmaster Peter Green told the Royal Commission into Institutional Responses to Child Sexual Abuse he believed the year 9 student had just been "rumbled" by other boys in the school's boarding house.

Trinity Grammar School deputy headmaster Peter Green after giving evidence at the royal commission on Friday. Credit:Janie Barrett

The Summer Hill school's headmaster Milton Cujes' evidence is he believed the allegation was "horseplay" based on the initial report from Mr Green and Robert Scott, then head of the boarding house where 60 boys resided.

The inquiry has heard a number of boarders were allegedly attacked at the school in 2000 by older students wielding wooden dildos and at least two boys were allegedly assaulted on up to 50 occasions.