Sexual touching was "seen as acceptable" at the prep school attended by Boris Johnson, an inquiry has heard.
The Independent Inquiry into Child Sexual Abuse (IICSA) has identified Ashdown House in Forest Row, East Sussex, as one of its points of focus.
Fiona Scolding QC said abuse was disclosed at the time by parents but the police were not informed.
The school had an "unforgiving" culture during almost 25 years of abuse from 1969, the hearing was told.
Ms Scolding said it was a "feeder school for the most significant public schools, such as Eton", with past pupils including the current prime minister.
Mr Johnson attended the school in 1975, six years after the first allegations of sexual abuse of pupils were made.
Ms Scolding told the inquiry: "The regime has been described in the [victims'] accounts as 'Spartan and unforgiving'.
"The boys who were abused identified a sexualised culture created by the abuse of staff against pupils.
"It was an environment where sexual touching was seen as acceptable and normal."
The hearing was told allegations of abuse had been made against at least seven Ashdown House staff, involving 15 complainants.
Two of those staff have since died, and one cannot be found, Ms Scolding said.
'Rotten to its very core'
Martin Haigh, who taught at the school between 1973 and 1975, is the only person to have been convicted.
Ms Scolding said: "Haigh admitted sexually abusing three boys to the [then] head teacher and left under a cloud in 1975."
But she said nobody thought to contact the police or refer the matter to the Department for Education.
Haigh was eventually jailed in March 2017 for 12 years for 11 sex offences against boys at the school and possession of indecent images of children.
Ms Scolding said boys at another school, St William's in Market Weighton, Yorkshire, were raped and sexually assaulted by members of staff.
She said the school, which closed in 1992, was "rotten to its very core".
At Sherborne Preparatory School in Dorset, head teacher Robin Lindsay walked around in his pyjamas "exposing himself, stinking of alcohol and tobacco," the barrister said.
Mr Lindsay was a "fixated paedophile" whose behaviour continued "unimpeded" for 24 years until he was banned from teaching, the hearing was told.
Founded in 1843, Ashdown House School has been owned by the Cothill Educational Trust since 2009.
The boarding and day school and now educates boys and girls aged from five to 13.
A number of other schools will be examined during this part of the inquiry.
The hearings are expected to last until Friday.