Durham police confirmed that five more men, aged in their 60s, have claimed they were attacked at The Chorister School at Durham Cathedral.
As in all previous cases, the men accuse Canon John Grove, headmaster from 1957 to 1978, of sexual abuse. He died in 2001.
The surge in new cases comes as police revealed they are now looking for witnesses and new victims.
A spokesman said: “The investigation is still in the process of identifying victims and other people who may be able to assist police. At this stage no other staff member has been identified as being involved in any aspect of abuse other than John Grove.”
Current school headmistress Yvette Day is also helping police by providing historical records.
Liz Dux, of law firm Slater Gordon, who is representing five abuse claimants, welcomed the police action.