Christopher Slater was jailed for more than four years in 2015 for abusing seven boys at the school over a 16-year period between 1976 and 1992.
Slater, now 71, returned to Leeds Crown Court today after pleading guilty to sex offences committed against a further victim who was also a pupil at the school.
The court heard the offences were committed in the late 1980s and early 1990s when the victim was in his early teens.
Richard Walters, prosecuting, said the offending was similar to the abuse committed against other pupils he was previously prosecuted for.
Slater, a classics teacher, groomed the victim and showed him pornography before moving on to committing sex offences against him.
The abuse took place at various places including the school and Slater's home.
The court heard the boy asked Slater for the abuse to stop but the teacher threatened to "out" the youngster as being gay.
Slater pleaded guilty to two offences of indecency with a child one of indecent assault.
The victim described in a statement, which was read to the court, how the abuse continued to blight his life.
He said: "I was ruthlessly exploited for his own sexual gratification.
"As a teacher in a leading boys' school he was in a position of trust.
"He groomed me until I was regularly engaging in sexual activity with him.
"His sexual offences scarred me, warping my sense of what humans were capable of.
"He ruined my ability to trust other people and drove a wedge between myself and my family.
"Twenty-five years on I almost never sleep through the night."
Benjamin Newton, mitigating, said Slater now suffers from dementia.
Mr Newton said the defendant also had depression and anxiety due to post traumatic stress disorder as a result of his time in prison.
Slater, now of Glamis Avenue, Heywood, Lancashire, was sentenced to four years, four months in prison in 2015 after admitting seven charges of indecent assault.
He was also prosecuted for child sex crimes in 2000, when he was jailed for three-and-a-half years for grooming and performing sex acts on an 11 year-old-boy.
Slater was given an 18-month prison sentence, suspended for two years, for his latest offences.
Judge Christopher Batty told Slater he would have imposed a longer prison term in 2015 had he then also been dealing with his latest offences.