The Demon Sword Master of Excalibur Academy (2023)

To protect the world from dangerous creatures known as Void, fifteen-year-old Riselia roams the land with her companions, defeating the monsters before they can do any harm. One day, she finds herself in an ancient tomb where she finds Leonis, a boy who has lost his way there. Without further ado, she takes him with her and decides to look after him like a big sister.

However, Leonis is no ordinary boy but the reincarnation of a feared demon lord who tried to subjugate the world a thousand years ago. When she entered his tomb, Riselia inadvertently interrupted his stasis, and now he has to find his way in a world completely alien to him. To learn more, he decides to enrol at Excalibur Academy to find out not only more about time but also about the new monsters he believes have a connection to his past.

Source: www.anisearch.com/anime/16676