Seiken Gakuin no Maken Tsukai

Seiken Gakuin no Maken Tsukai summary:

In the fantasy story, the demon king Leonis sealed himself away for 1,000 years in order to prepare for the final battle, but when he woke up, he found himself in the body of a 10-year-old boy. A beautiful young woman named Lisellia from the Holy Sword Academy then says to him, "Why were you locked up in such a place? It's OK now, this big sister will take care of you!" Under Lisellia's care, Leonis is shocked to see that the world has completely changed. Bewildered and confused, Leonis finds himself enrolling at the Holy Sword Academy.