Yakuza Cleaner is a Manga/Manhwa/Manhua in (English/Raw) language, Action series, english chapters have been translated and you can read them here.
I’m Banjo Taiga, and as of today, I’m assigned to this school as the cleaner. Banjo Taiga is a child of the Yakuza era. An expert at killing who has been trained in the underworld for as long as he can remember. However, the Yakuza being an overwhelming presence in the underworld is a thing of the past. Gokudō is now a relic left behind by the times, and the Yamada-gumi, to which Taiga belonged, has been dissolved and reborn as a respectable company. His new job is as a cleaner. The genius of killing has become a cleaner. His workplace has gone from the underworld to a school. Now, fighting and violence are forbidden for a reputable employee, let alone murder. However, what awaits Taiga is a high school full of delinquents with rampant bullying, violence, smoking, drinking, and all sorts of problems. A situation that even teachers and other adults can’t handle. “I’ve come to clean up this filthy school.” Numerous splendid techniques and refreshing developments. Making full use of the skills he mastered to survive in the underworld, the former yakuza’s “filth” cleaning is about to begin!