Mahou Shoujo Cthulu

Yuzuki, a tall but soft-spoken girl, is in love with Akuta, a boy in her class. However, when Akuta was found after school standing in the school hallway wearing a magical girl outfit, he begins to be bullied. Wanting to help Akuta after someone threw a pie at him, Yuzuki chases after him and sees him face to face with a half-fish man.

(Source: Rookie Shonen Jump, translated)