The incident occurred all of three years ago after a male pupil had filled the blackboard in the class with sexual swear words and sketches depicting scenes of nudity.
The teacher, 47, claimed that she had only revealed her breasts in reaction to the boy's provocative doodles and that there was nothing sexual about the act.
"I just snapped. I was trying to get him to stop writing and drawing filth on the blackboard," the teacher told the court, newspaper Norra Västerbotten reports.
Taking account of the fact that the woman had been made redundant from her job, the court limited her punishment to a fine equivalent to 60 days' wages. She was also ordered her to pay her former pupil 5,000 kronor ($760) in damages.
The woman was cleared of two further charges after two other boys claimed that she had grabbed their backsides and genitals as she made her way into the classroom.
The former teacher admitted that she may have pinched the two boys when they blocked her path as she attempted to pass but the court found the pupils' witness testimony to be unreliable.