Ichinensei ni Nacchattara

Also known as: 一年生になっちゃったら, In The First Grade, Elementary School, If he Becomes in the First Grade of Elementary School

by Masakazu Ooi

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)