Shitei Bouryoku Shoujo Shiomi-chan

"Gen Shiomi, the 56-year-old boss of the Hitotsubashi mob syndicate, must go into hiding to protect his life from potential assassins. He opts to get plastic surgery, only to end up looking like a teenage girl, but he resolves to stay like this until his life is no longer in danger. However, he finds it hard to not be suspicious, but he is saved from being the target of police by a teenage boy named Tatsuya Umemiya. After helping Tatsuya out of some troubles of his own, Gen is forced to enroll into junior high school as ""Shiomi Iwao"", only to discover that one of his new classmates is Tatsuya." (Source: Mangamonster.net)