The house arrest handed a former Calgary-area educator for molesting three boys decades ago was insufficient punishment, Alberta's top court said Tuesday.

A three-member Alberta Court of Appeal panel instead sent Fred Henry Archer to a federal prison to complete what should have been a three-year sentence.

Because Archer, 65, has already done five months of his house arrest, he must serve two years and seven months of jail, the appeal court said.

The judges rejected defence counsel Balfour Der's submission Archer's age, and other factors, warranted sparing him jail.

Der noted Archer has successfully taken intensive counselling, had no other criminal involvement and committed no offences since he assaulted his last victim almost 20 years ago.

Last October, Archer pleaded guilty and admitted sexually abusing the boys while working as a guidance counsellor at Springbank Middle school in the 1980s and early 1990s.

After a teary apology in which Archer admitted he took advantage of vulnerable young boys, provincial court Judge John Reilly handed him the maximum conditional sentence of two years less a day.

"I tried to help them have a better life, however I failed," Archer told Reilly at the time.

"I have hurt these young men and damaged their lives through my actions," he said.

"I know it sounds foolish to say that I never meant to hurt any child, but I did nonetheless."

The appeal court agreed with the Crown prosecutor's written submission Reilly's sentence was demonstrably unfit.

In a rare move, the judges did not ask for prosecutor Goran Tomljanovic to make oral submissions on his own appeal.