Daisanji Seichouki, Ootsuka-kun!

In 1987, Tsuyoshi Ootsuka fainted in a freak accident. Twenty-six years later, he miraculously wakes up. Now it's time for Ootsuka to go back to school where he plans to live his youth with full force! There's just one problem: on the outside, everyone sees a 40-year-old man, while on the inside, he's still just a 14-year-old boy.
(Source: Gangan Online)