Gary Dobbie was found guilty last month of four counts of indecent assault against a boy between 1986 and 1991, having already been convicted last year of abusing eight other pupils.
The 67-year-old was a teacher and chaplain at Christ's Hospital School in Horsham, West Sussex when the assaults took place.
He was sentenced at Hove Crown Court on Wednesday to 12 years in jail for all 19 offences, a court official said.
He is one of five ex-teachers convicted after police investigated complaints spanning 30 years by 22 of the school's former students.
Following the sentencing current headmaster Simon Reid said: "It is hard to find the right words to express the depth of our sadness and dismay, as a school community, at the dreadfulness of this situation.
"We are deeply sorry that the school failed to protect the victims of these offences and those others who were entrusted to our care during what should have been happy and settled years.
"Our apology to them is heartfelt and we hope that this conviction, the culmination of what must have been a very difficult experience in the criminal justice process, will bring some measure of comfort.
"The school today is a very different place, one in which the safety, happiness and wellbeing of our pupils is at the centre of all we do.
"Close links with local statutory agencies help ensure that our practices and decision-making are compliant, effective and open."
In the latest trial, Dobbie's victim told jurors it was "habitual" for him to be groped during night visits to his on-site lodgings while he was given wine and once fed smoked oysters.
Dobbie preferred to smoke cigars but would buy Turkish cigarettes for his pupil and also assaulted the boy during two school trips, at one point drunkenly lunging at him in a violent and sexual manner.
Describing him as "a very civilised man" who loved indulging in the grandeur of highchurch traditions, the man said the whole school would attend Dobbie's services in the chapel four or five times a week.
He told how visiting Dobbie was an "escape" from being bullied.
But he was shown little sympathy and instead groped under his school shirt while they sat alone on a sofa.
The married father, who still battles with urges to suppress the traumatic memories, said when he confided in a fellow teacher at the school he was told to "get over it and move on".
Dobbie was arrested while teaching at independent Shrewsbury School, Shropshire, where he worked after leaving Christ's Hospital. He then moved to France after living in Hereford.
He was found guilty last year of 15 charges detailing multiple offences against six boys and two girls as young as 12 between 1998 and 2001.
He had denied 12 counts of indecently assaulting four boys and two girls, attempting to indecently assault a boy and two counts of indecency with a child.
Sports coach Ajaz Karim, was jailed in August for 10 years after assaulting six girls between 1985 and 1993.
In July, James Husband, also known by his middle name of Andrew, was jailed for 17 years for raping and indecently assaulting a girl as young as 14 between 1990 and 1994.
Peter Webb and Peter Burr admitted indecently assaulting pupils at the school between the 1960s and 1980s and were also jailed.
Christ's Hospital is one of the oldest boarding schools in the country, dating back to the 16th century, and the Queen is a patron.
Pupils still wear a Tudor-style uniform of a long blue coat and high yellow socks with boarders charged fees of up to £31,500 a year.
Poet Samuel Taylor Coleridge and Sir Barnes Wallis, inventor of the bouncing bomb, are counted among its famous alumni.
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