TSF Monogatari

Takumi Musashino was once an ordinary high school boy, but that changed when his body transformed into that of an alluring high school girl thanks to an experimental gene therapy treatment he received after being diagnosed with a terminal illness. She astonishes her classmates with her new body, receiving mixed reactions from them. While girls like Anri Sogahara offer her important adjustment advice, her male peers begin to see her as an object of their desires...

Although Takumi is both confused and nervous about her new being and appearance, she will soon discover the wonderful pleasures of being a young, adolescent woman.

(Source: MAL Rewrite)