Himitsu no Mizuki-sensei

Tenma Akitaka, a curious junior high school boy, has something he can't stop thinking about. It's his homeroom teacher, Mizuki-sensei. Her glasses and dull outfit gives her a plain appearance, but for some reason she always visits the old school building with her cute face! Tenma's intellectual curiosity gets the better of him and he decides to investigate her secret. However, the more he investigates his sensei, the more her naughty and airheaded happenings are revealed to Tenma! He is supposed to be investigating her, but all he can do is satisfy his H-seeking curiosity! Can he keep up with Mizuki-sensei's naughty happenings and explore her secrets?

(Source: Kadokawa, translated)