A judge has ruled that Michelle Francouer is not guilty of sexually assaulting a 15-year-old boy. (CBC)

A Shell Lake teacher has been found not criminally responsible due to mental illness in the sexual assault of a 15-year-old boy.

Michelle Francoeur was charged following a complaint she had an inappropriate sexual relationship with a boy who was formerly a student at the school where she taught in 2008.

But on Friday a Saskatoon judge found her not criminally responsible, finding Francoeur suffered from a bipolar disorder.

After a joint submission by both the Crown and defence lawyers, Court of Queen's Bench Judge J. Duane Koch gave Francoueur an absolute discharge. Court was told Francoeur, aged 37 at the time, began having health problems in June of 2008 and was at first wrongly diagnosed with depression. Later she was diagnosed as having a bipolar disorder.

It was a "very tragic circumstance," Francoeur's lawyer, Aaron Fox, told CBC. "It was a lady who has an underlying condition which was misdiagnosed and received medication that really had a catastrophic effect on her."

The boy, who initiated contact via text messages with the teacher, had transferred to another school at the time of the relationship and was never a student of Francouer's, court heard.

Francouer, who taught at a kindergarten to Grade 9 school in the village of Shell Lake, Sask., had been charged with sexual touching, sexual exploitation and sexual assault.