Noir Soft - Sei Hanten Shou no "Ore" ga "Watashi" ni Naru Made

According to the doctor, the protagonist has a unique constitution that will completely change him into a girl in a year. Since he can't go back to how he used to live before, he must move to a girl school and learn how to be a girl.
However, his body is currently still that of a boy. So until he completely changes gender, he has to wear girl clothes and continues his school life without exposing his secret.