Veteran Shougakusei Takashi

Failing to complete the class goal "Leave no food behind!" for 11 years straight, Takashi Mochizuki (4th grade) has turned 20 stuck as an elementary schooler. Then, an old classmate of Takashi's returns to his current class, Takashi's first love, Megumi Asahina (20), as none other than his homeroom teacher!

(Source: Seikaisha, translated)