Chuusotsu Worker kara Hajimeru Koukou Seikatsu

Makoto Katagiri is 18 and works at a factory. He wants to get back at everyone who laughs at him for not having gone to high school, so he enrolls in a melting pot of a correspondence school with his little sister, Maaya, who failed her high school entrance exams. On the day of the entrance ceremony, he meets Rio Aizawa, a beautiful lady, in a dramatic fashion...

(Source: Pocket Comics)