A three-judge panel sentenced Neil Bantleman, of Burlington, Ont., to 10 years in prison.
"Today is completely a miscarriage of justice," Bantleman told The Associated Press. "I'll keep fighting."
The court also ordered Bantleman, 45, to pay a fine of nearly $10,000 or serve six additional months in jail.
"The defendant has deliberately committed violence and obscene acts," the judges said in their verdict.
It was a long day for Bantleman's wife, Tracy, who was exhausted at its conclusion.
She told The Canadian Press by email that she is both outraged and heartbroken by the decision.
"My husband is innocent," she said from Jakarta.
"He always has been and always will be."
Bantleman's brother, Guy, said the case has been "shrouded in secrecy."
"The judge basically threw out every piece of evidence, every witness that the defence put up through the case."
Guy Bantleman added the verdict and sentence "falls into line with everything we've had to go through with this entire case" and said he believed this was "merely a hearing to get to a guilty verdict."
"It's mind-boggling," he said. "There hasn't been a piece of evidence that has actually proven anything."
In December, five janitors from the Jakarta International School school arrested in the same case were sentenced to up to eight years in jail. Police said a sixth suspect in that group committed suicide while in custody by drinking bathroom cleanser.
The trial of another defendant, Ferdinant Tjiong, an Indonesian teaching assistant, is continuing at the same court.
Children of foreign diplomats, expatriates and Indonesia's elite attend the Jakarta International School. It has 2,400 students aged three to 18 from about 60 countries.
The school's principal and a number of other teachers have said they believe Bantleman is innocent.
Prosecutors told reporters after a hearing last month that they sought heavy sentences because as teachers, Bantleman and Tijong had traumatized the victims.
Now Tracy Bantleman faces a decade of life with her husband behind bars.
"We have done nothing but give the best of ourselves to the world," she said.
— With files from Alan Black in Toronto and The Associated Press
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