Shitei Bouryoku Shoujo Shiomi-chan

A 56-year-old mob boss named Gen Shiomi goes into hiding after becoming a target for assassination. He undergoes plastic surgery, which results in him appearing as a teenage girl, and decides to remain in this form until it is safe to return. While struggling with the new appearance, he is protected from police suspicion by a boy named Tatsuya Umemiya. After assisting Tatsuya with his own problems, Gen is compelled to attend junior high school under the alias "Shiomi Iwao." There, he finds out that one of his classmates is Tatsuya.