18+ Ichinensei ni Nacchattara

While on his way to school to confess his love to classmate Kusanagi Mikuru with a love letter, high school ninensei (2nd year in highschool) Takatou Iori gets run over by a truck to save a cute elementary ichinensei (1st year in elementary school) schoolgirl.

But when he wakes up, he's revived by a perverted doctor, as a full fledged female elementary ichinensei student and is now in the same class as the girl he saved.

(Source: MangaHelpers)