John McMillan assaulted the boy in a tent on four nights during a trip to Gippsland when he worked at prestigious Wesley College in 1975.
McMillan, now 72, went on to sexually assault another boy twice at progressive Kew school Preshil in 1991.
McMillan washed the 12-year-old’s genitals in a gym shower while they were both naked in 1991, then rubbed his thigh during recess the following year.
McMillan was found guilty of four counts of indecently assaulting a child, performing an indecent act on a child and behaving offensively in a public place.
He was given an 18-month sentence, of which 14 months were suspended, at the Melbourne Magistrates Court on Wednesday.
six charges, would serve four months in prison.
However, he walked away from the court after being granted bail pending an appeal at the County Court on October 6.
Former Preshil student Lisa Sinha said the outcome must have been “shattering” for the “brave men” who had received “life sentences” due to McMillan’s abuse.
“I was relieved to see him get justice and then immediately disappointed to see him walk free again,” she said.
Ms Sinha, who went to the school in the 1960s and 1970s, said another alleged perpetrator, who is believed to be dead, abused her friends on school camps.
She accused Preshil of being slow to act and defensive in response to a flood of allegations about the two men that emerged on social media in 2013.
“I would have thought Preshil could have been a leader,” she said.
Mr Lethbridge said McMillan’s crimes had caused destructive and lasting effects on his victims.
“The crimes have devastated their lives and continue to significantly impact them,” he said.
“You were a person trusted to care for children and given authority over them. You betrayed that trust and abused that authority.”
Mr Lethbridge noted the abuse of the Wesley student took place in an isolated tent where he was “effectively powerless”.
“There was, in short, no escape,” he said. “This happened repeatedly.”
He said McMillan, who was deregistered from teaching in 1996, deserved a prison term, but took his ill health into consideration when sentencing him.
McMillan, who sat in the court’s front row with a walking stick by his side, rocked his head back and let out a sigh at the magistrate’s first mention that a prison term was warranted.
Defence counsel Dermot Dann, QC, asked Mr Lethbridge to take into consideration his client’s “outstanding” contribution to the community, including belonging to choirs, volunteering at the CFA and involvement in an olive growers’ association.
“This man has contributed to the community very well and he’s been out of trouble,” he said.
Mr Dann said his client’s guilty plea, lack of prior offences, low risk of reoffending and ill health should also be considered, suggesting either a community corrections order or suspended sentence was appropriate.
McMillan’s case was adjourned in March when a broken leg prevented him from travelling to Victoria from his home in Queensland.
Mr Dann said McMillan suffered from a heart condition, osteoporosis, scoliosis and psychological issues, including adjustment disorder and a preoccupation with his medical condition.
“A term of imprisonment would be particularly burdensome for a this man at his age,” Mr Dann said.
Prosecutor Luisa Di Pietrantonio asked that McMillan be jailed, noting the maximum five-year prison term available.
McMillan was placed on the lifetime sex offender registry.
As he sat in the dock ready to be led away by police, his two now-adult victims shared brief words and a few smiles with each other.
Another former student, Jason Downing, who was taught by McMillan in the late 1980s, said he believed the sentence was fair due to the nature of the offending as well as his age.
Mr Downing said McMillan would regularly order students to take off their clothes in a communal shower after gym lessons.
The teacher would shower with them, also naked.
“He’d be lathering himself up. He’d put his towel between his legs. It was reasonably perverted,” he said.
Although Mr Downing didn’t think anything untoward was going on at the time, he said he wasn’t surprised when the allegations surfaced, including by some of his friends.
“He was generally not well liked. He was a bit of a weirdo,” he said.
In a letter sent to parents at the school on Wednesday afternoon, Preshil offered victims its “wholehearted” support.
“Preshil has apologised publicly for the crimes of this man, recognising that we do not know the full extent of the pain and damage he has caused,” the letter, signed by principal Marilyn Smith and school council chair Andrew McMeekin, said.
“The school wishes publicly to acknowledge the courage and the patience of those who have not allowed this matter to rest.”
The letter also acknowledged some of the school’s community were disappointed that its communication about McMillan had not reached all alumni.
Preshil was the creation of founder Greta Lyttle, and her niece Margaret Lyttle, who was principal for 50 years, until 1994.
Its alumni include Olympic gold medallist Lauren Burns, comedian Gina Riley and world-renowned philosopher Peter Singer.
Wesley College was a boys school before 1978, when it first opened enrolment to girls.
The Uniting Church school’s notable alumni include former prime ministers Sir Robert Menzies and Harold Holt, Olympic gold medallist Michael Klim and tennis player Mark Philippoussis.