Koster has already been jailed twice - in 2003 and 2012 - for indecent treatment of two boarders at the Queensland school where he was a teacher.
Those offences, against boys aged about 10, occurred in the 1980s, according to court documents.
Koster was dismissed from the school after being confronted with allegations and making full admissions in 1984.
But nothing more was done until 2003 when he was charged with offending against another boy.
In the meantime Koster continued to work as a teacher, giving up that career in 1992 to study theology, social science and journalism before working as a bookkeeper.
But for years until 1996 he also sexually abused another boy from the age of eight, the Brisbane District Court was told on Monday.
Koster performed oral sex on the child, ejaculated in front of him and offered financial incentives.
On the last occasion the boy, then a teenager, punched Koster in the face after waking to the man licking him.
Koster pleaded guilty to eight offences including maintaining a sexual relationship with a child, indecent treatment of a child and sexual assault.
His case was unusual in that he had not offended for 26 years, Judge Paul Smith said in sentencing Koster on Monday.
The now-70-year-old has been diagnosed with pedophilic disorder due to his recurrent and intense fantasies of children under 13.
"This sexual orientation resulted in depression and heavy drinking," Judge Smith said.
Koster was sentenced to four years and six months in jail to be suspended after serving 15 months.
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