He was in High School during the Vietnam War. At the time, a constitutional amendment had recently that allowed 18-21 year olds to vote and run for office. At some point, Michael Moore got fed up with his principal and wanted to get rid of him. He decided that the best way to get rid of his high-school principal was to run for office, become his boss and fire him. He ended up winning and 9 months later, he did just that. 

He did a whole lot of other things while in office. In fact, 2 years after he won, there was a special election called to vote on whether or not he should be removed from the board of education. Check out his story here