Ichinensei ni Nacchattara

While on his way to school to confess his love to classmate Kusanagi Mikuru with a love letter, a second year high school student Takatou Iori gets run over by a truck to save a cute first year elementary schoolgirl. But when he wakes up, he's revived by a perverted doctor, as a full fledged female first year student and is now in the same class as the girl he saved.