Akishima Suicide☆Club

16-year-old Sakura Ryousuke lives a relatively ordinary life, but he is disgusted by the unfairness of the world. One evening, he sees a girl dressed up as a 'magical girl,' breaking up a gang beating on one of the toughest guys in school, Shishido Haruo. He soon works out that she is actually his quiet classmate, Kakinoue Suu, and that she and Haruo (and Haruo's little sister) are part of a team that she calls the Akishima Suicide Club. They are dedicated to the death to protecting the city and changing it for the better. Ryousuke joins their team, and the ASC continues risking their lives against the dangerous thugs and criminals.

(Source: MangaHelpers)