World Teacher: Isekaishiki Kyouiku Agent

Sirius was the best secret agent the world had ever seen. But after being killed in action to save his comrades, he is reborn in a strange new world. Literally! The only regret from his previous life was that he didn't get to see his students grow up. So, with his knowledge and experience from his previous life, Sirius sets out anew to travel this strange world, changing the lives of those he meets along the way! To some he's a beloved teacher, to others he's a strong role model, or even a crush! "If you have nothing to lose, then come with me. I'll teach you how to live again." The once strongest secret agent will now travel the world with his students! His journey has only just begun... (Source: Tentai Books)