Ichinensei ni Nacchattara

As a 2nd year high school student, Takatou Iori was on his way to school with a love letter for his crush, Kusanagi Mikuru, when he stumbled upon a dangerous situation. He heroically rescued a cute 1st year elementary school student from getting hit by a truck but got hurt himself in the process. After waking up from the incident, he discovered that an unscrupulous doctor had revived him as a 1st-year female student in the same class as the child he rescued.