In The First Grade

While on his way to school to confess his love to classmate Kusanagi Mikuru with a love letter, high school 2nd-year Takatou Iori gets run over by a truck to save a cute elementary schoolgirl.

But he wakes up, revived by a perverted doctor, as a full-fledged female first-grader, and is now in the same class as the girl he saved.