Paradise of Innocence

Paradise of Innocense (無邪気の楽園, Mujaki no Rakuen) is a Japanese manga series by Uran. An OAD series was bundle released with the 6th, 8th and 10th manga volumes.

Plot
Shouta, a 25-year-old NEET, is mocked by his former female classmates who've all become successes, at their class reunion. A mysterious incident at the school pool throws him back in time and lets him deal with those same classmates back when they were fifth grade girls.