Otona Mitai ni Koishiteru!(2014)

Otona Mitai ni Koishiteru!

A primary school student transforms into a high school girl with a mysterious drug!?

Nana, a fifth grader, can transform into a beautiful high school girl by taking a drug that turns her into an adult! She comes up with a plan to get back at her mean tutor Youichi, but...? A high-tension romantic comedy about the naive and foolish Nana and the tsundere, bespectacled Youichi who can't be honest with himself.

(Source: Shogakukan, translated)