Seiken Gakuin no Maken Tsukai

the demon sword master of excalibur academy - One thousand years ago, Leonis Death Magnus, undead sorcerer and the greatest of the Dark Lords, entered magical stasis during a climactic battle. When he awakens, he finds a beautiful silver-haired young woman named Riselia standing before him. More shockingly, Leonis has been transformed into a ten-year-old boy! Now he'll have to adapt to this strange era and deal with the twisted monsters called Voids—all while keeping his true powers a secret from his lovely new guardian.

(Source: Yen Press)

Note: Each episode streamed 1 week early on ABEMA. The original TV broadcast started on October 3rd, 2023.