Tada no JK, Isekai de Maou ni Naru

Airi Katagiri, is a high school girl who cannot ignore people in trouble. One day while on her way to school, a magic circle appears under Airi's feet and she is sent to the Demon World. In the Demon World, demons are performing a ritual to revive the Demon Lord, which summons Airi. Seeing her, the demons think that they have failed to revive the Demon Lord, but it turns out that Airi has the power of the Demon Lord. In order to find a clue to return to her original world, she decides to become the Demon Lord.

(Source: Natalie, translated)