Seiken Gakuin no Makentsukai (light novel)

The Demon Sword Master of Excalibur Academy (聖剣学院の魔剣使い, Seiken Gakuin no Makentsukai) is a Japanese light novel series written by Yū Shimizu and illustrated by Asagi Tōsaka. Media Factory have published six volumes since May 2019 under their MF Bunko J imprint. The light novel is licensed in North America by Yen Press.

Synopsis
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.