A former Calgary guidance counsellor is going to prison for molesting three students in the '80s after Alberta's top court overruled his original conditional sentence.

Fred Henry Archer, 65, pleaded guilty in October 2009 to two counts of sexual assault relating to the abuse of three boys while he was a counsellor at Springbank Middle School, west of Calgary.

Provincial court Judge John Reilly gave him a conditional sentence of two years less a day.

However, on Tuesday, the Alberta Court of Appeal replaced that sentence with a three-year prison term, calling the conditional penalty "demonstrably unfit," according to the Calgary Herald.

The court gave Archer credit for five months of house arrest, and he must now serve the remaining two years and seven months behind bars, said the report.

Archer was retired when he was arrested in 2008, but had worked occasionally as a substitute teacher.