From Wikipedia:

Set in the modern age, the story is about Miharu Rokujou, who just wanted to live a normal life and inherit an Okonomiyaki restaurant. At the moment, his biggest problem was the constant pestering from Kouichi Aizawa and Durandal Thobari Kumohira, who started a Nindō (忍道?, lit. "Way of the Ninja") Club. Miharu wanted nothing to do with "ninjas" and turns them down every time. But soon, he is attacked by an actual group of ninjas and is protected by Kouichi and Kumohira. There, Miharu learns of the hidden ninja world Nabari, the roles of his classmate and teacher in that world, and of the secret art Shinra Banshou (森羅万象 Shinrabanshō?, lit. "all things in nature") within him. The secret art, containing all the world's knowledge, is sorely coveted in Nabari. And because of that, Miharu cannot return to his normal life. He must become the "Ruler of Nabari" to survive.

From Baka-Updates:

Rokujou Miharu was a seemly reluctant junior high student living in his own world. However, within him, lurked the ultimate power of Nabari, “Omnipotent”. To possess such supreme weapon, the Wolf Pack of Iga swore to bring Miharu back to Iga village. On the other hand, Miharu’s English teacher, Kumohira sensei, and classmate, Kouichi, were the Banten ninjas whose mission was to protect Miharu. A war waged over the ownership of “Omnipotent” thus started. To survive, Miharu must enter the hidden world Nabari, to become the king.