Harukanaru Toki No Naka De 4 summary:

Ashihara Chihiro is a regular high school student. One day a man came to her and told her that she's actually a princess from another country in another dimension. Chihiro later found out that it was true, and in fact her own best friend and teacher were actually her guardians! Her country has been taken over by Raja, who is now king. Chihiro and her guardians come back to that dimension to take back their country. In her quest, Chihiro met other guardians and warriors who were willing to help her achieve her goal.