Yankee-kun to Megane-chan [Manga]

Daichi Shinagawa was just a Yankee (Japanese term for delinquent) who wanted nothing to do with his high school life. Hana Adachi, the dedicated class representative, however would not allow it and constantly bothers him to be involved in school life. Shinagawa is first confused on why does she keep pestering him until he discovers her secret.

While she may look like a typical class representative stereotype, Adachi is actually not very smart and lacks common sense, and Shinagawa eventually learns that Adachi is a former delinquent. Regretting being a yankee during her middle school years, Adachi decides to change her ways to achieve her dream of becoming the best class representative. She decides to help Shinagawa so he will not continue to make the same mistake that she did. Thus begins the adventures of these two unlikely friends and their classmates.
(-from wikipedia)