Heian Igaku Shoujo Ibun

Hina, a high school girl aiming to become a doctor, slips into the Heian period during a school trip! There, she is asked to take the place of Minamoto no Yoshitsune! Her only source of comfort is Taira no Koremori, a member of the Taira clan, who are enemies of the Minamoto clan! (Source: Kadokawa, translated)