Story here. Perhaps she was mentally deficient at the time, but given the description of her actions, I am finding it hard to believe she didn't know what she was doing is wrong. It's not like she made one mistake. She made many over a long period of time. Excerpt:

'SASKATOON — Family and supporters of a teacher from Shell Lake, Sask., sighed with relief in a Saskatoon courtroom Friday after a judge declared her not criminally responsible, due to mental illness, for a sexual relationship with a 15-year-old former student.

Michelle Francoeur was in an extreme “manic state” and lacked the capacity to make rational decisions when she agreed to have sex with the teen boy on several occasions between Sept. 1 and Nov. 20, 2008, Queen’s Bench Justice Duane Koch found.

“The criminal law does not want to punish people who were suffering a mental disorder at the time of the act,” Crown prosecutor Mitch Piche said outside court.'