From Kodansha:
Watching the news, Takemichi Hanagaki learns that his girlfriend from way back in middle school, Hinata Tachibana, has died. The only girlfriend he ever had was just killed by a villainous group known as the Tokyo Manji Gang. He lives in a crappy apartment with thin walls, and his six-years-younger boss treats him like an idiot. Plus, he's a complete and total virgin. At the height of his rock-bottom life, he suddenly time-leaps twelve years back to his middle school days! To save Hinata, and change the life he spent running away, hopeless part-timer Takemichi must aim for the top of Kanto's most sinister delinquent gang.

Original Web Manga

Digital Volumes English: Kodansha
Individual Chapters English: Azuki, INKR