Tetsu-Kabuto (lit. Iron Helmet), known as Ninja in the anime, is one of the instructors at Otokojuku.

(Source: Sakigake!! Otokojuku Wiki)

Otokojuku - a private school for juvenile delinquents that were previously expelled from normal schools. At this school, Japanese chivalry is taught through feudal and military fundamentals. Similar to an action film, the classes are overwhelmed by violence. Only those who survive it become true men. <br><br>
(Source: Anime News Network)

Otokojuku is the kind of school that seems like it's up to no good. For most of their students, this is the only place that will take them despite or even because of their violence issues. Headmaster Heihachi Edajima believes that giving young men a military education will provide them with brighter futures, but how much is wishful thinking?<br>
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(Source: Manga Planet)