Yen Press The Demon Sword Master of Excalibur Academy, Vol. 1 (light novel)

"""One of the most powerful demon lords in history has been sealed away for a thousand years. When he finally awakens, he is reincarnated as...a ten-year-old boy?! Now under the care of the girl who released him, the demon lord Leonis must navigate the perils of Excalibur Academy!""-- " ...