The teacher was filmed by Toni Vesikko, 15, who loaded the video on to YouTube. He named her and said she was "a lunatic singing at the karaoke of the mental hospital".
More than 600 people had seen it by the time it was removed.
Last week Nurmes district court found Vesikko guilty of intentional defamation and fined him pound;60. He was ordered to pay pound;500 in damages for "causing harm and suffering" and pound;1,000 court costs.
The court said he had "falsified facts about the institution and the teacher's mental state".
Compare schools to asylums at your peril.