Fred Henry Archer pictured in a 23-year-old yearbook photo. ((CBC))

RCMP have charged a former school guidance counsellor with a sexual assault alleged to have occurred more than two decades ago.

Fred Henry Archer, a 64-year-old Calgary man, was arrested earlier in the week and charged with gross indecency and sexual assault, Cochrane RCMP announced on Thursday.

In a press release, police said a man came forward with allegations that he was sexually assaulted in 1985 when he was a Grade 8 student at Springbank Middle School, west of Calgary.

Archer was a guidance counsellor at the school at that time. Police say he worked "continuously" in other Calgary-area schools, although they aren't naming the schools.

Archer was recently working at the Calgary Science School, a charter school, said Supt. Gordon Welch.

Welch released a statement to the media Thursday, saying Archer worked part time at his school in "various capacities" since 2002, but was "suspended pending a legal decision on this matter."

"We at the Calgary Science School are devastated to learn of this. We have taken appropriate steps to inform our staff, parents and students of this information. Counselling services will be made available to students and staff who require some help coping with this matter."

RCMP spokesman Sgt. Patrick Webb says investigating historic cases can be more difficult than recent ones because a lack of physical evidence means police have to focus on statements from witnesses recalling something that happened decades ago.

Archer has been released from jail and told to stay away from anyone under age 18. He also has restrictions on being in public places where children are present.

He is set to appear in Cochrane provincial court on Nov. 20.

With files from Peter Akman