The Independent Inquiry into Child Sexual Abuse (IICSA) said there was a "sadistic and predatory" atmosphere at St Benedict's School, Ealing, in the 1970s when some of the abuse began.
In a damning report covering the independent day school and the adjacent Ealing Abbey, the IICSA said significant opportunities to stop abusers in the past were not acted upon.
The report highlighted failings by school leadership, police, the Crown Prosecution Service (CPS) and child protection teams to help stop the abuse and convict abusers.
The inquiry was told of the "cold, grim, forbidding" atmosphere at the school in the 1970s that made attending it "terrible" for children.
"There was a culture of excessive corporal punishment. Physical abuse in many cases was used as a platform for sexual gratification, and a means by which to instigate sexual abuse," the report said.
"Corporal punishment was also used to punish boys who sought to protect themselves and others from sexual abuse."
Staff members described reporting concerns as difficult due to a "mafia"-like atmosphere and feared losing their jobs due to the seniority of those complained about.
The report highlighted that since 2003 two monks, Laurence Soper and David Pearce, and two lay teachers, John Maestri and Stephen Skelton, have been convicted of multiple offences involving the sexual abuse of more than 20 children between at least the 1970s and 2008.
In 2017, Soper, who held a series of senior roles at the school and abbey, was jailed for 18 years for a string of sex attacks against boys in the 1970s and 1980s.
Pearce, once head of the junior school and then bursar, was jailed in 2009 after he admitted 11 charges of indecent assault dating back to 1975.
Between 2003 and 2009, Maestri admitted five indecent assaults against children at St Benedict's in the 1970s and 1980s and was jailed.
Skelton was convicted in 2010 of indecent assaults against two complainants said to have occurred in 1983 at St Benedict's.
The IICSA report said it had received evidence of at least 18 further allegations against the men and eight other monks and teachers.
This ranged from corporal punishment to grooming, fondling of genitalia, masturbation, and oral and anal rape.
"The true scale of sexual abuse of children in the school over more than 40 years is unknown," the report said.
The actions of Abbot Martin Shipperlee, who took over the role from Soper in 2000, to tackle concerns were labelled "frequently inadequate, ineffective and ill judged" in the report.
An example cited included a lack of extra restrictions being placed on Pearce following a civil trial in 2006 establishing his abuse.
Abbot Shipperlee resigned from his role during the inquiry's investigation, acknowledging "serious failings" under his leadership.
The IICSA report, which was informed by five days of public hearings held in February, said the advice Shipperlee received from the Diocese of Westminster child protection team was often "flawed".
Christopher Cleugh, headmaster of the school from 2002 to 2016, was accused in the report of minimising questions of child sexual abuse to teachers, external bodies and parents "to the point of materially misrepresenting significant facts".
Mistakes by the Metropolitan Police in investigating early allegations against Pearce and Soper were criticised in the report, while the CPS was said to bear responsibility for the lack of prosecution of the pair in 2004 when allegations were made.
The report also criticised the Independent School Inspectorate's 2009 inspection of St Benedict's that concluded its child protection policy was compliant with statutory guidance and that an independent review of Pearce's offending had been conducted and its advice implemented.
It took members of the public to point out these conclusions were wrong before the report was withdrawn in April 2010.
The Charity Commission's first inquiry into Ealing Abbey's handling of Pearce between 2006 and 2008 was found in the report to be "deficient".
Its review, which took place during the period when Pearce was abusing, concluded he was being managed appropriately.
The report said the commission "failed to scrutinise or test" assurances given by Ealing Abbey.
Once an all boys school, St Benedict's became co-educational in 2008 and accepts children from nursery age to 18 years old.
The number of monks teaching at the school has varied over the years, but as of 2019 there are none in teaching roles.
Governance of the school was taken out of the hands of abbey monks following the establishment of a new charitable trust in 2012 following the recommendation in a report by Lord Carlile of Berriew QC the year before.
School headmaster Andrew Johnson, appointed in 2016, was described as making a number of improvements to safeguarding, including record-keeping and vetting.
The new report forms part of the IICSA English Benedictine Congregation (EBC) case study, within the wider investigation into the Roman Catholic Church.
It highlighted themes in the institutional response to child sexual abuse by the EBC, describing a "closed, defensive and resistant" culture.
"Often teachers and other monks would be disinclined to believe that a monk could perpetrate such abuse," the report concluded.
"They were reluctant to support complaints for fear it would undermine the institutions and the Church."
The report added: "It remains to be seen whether Ealing Abbey proves itself capable in the future of ensuring proper safeguarding of children at risk."
The report noted that a request was sent to the Holy See, initially through its diplomatic representative in the UK, the Apostolic Nuncio, for a witness statement covering questions on what steps were taken after Soper disappeared from the country in 2011 that might have helped find him.
The Holy See declined to provide a statement, a move John O'Brien, secretary to the inquiry, described as "regrettable".
Summing up the findings in the report, Mr O'Brien said it identified "a toxic culture within the school exacerbated by failures by outside agencies to properly carry out their respective roles that would have stopped this abuse in its tracks far sooner had they done so".
Recommendations from the investigation into the Roman Catholic Church will be published at a later date.
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