Tenseishite Shota Ouji ni Natta Kensei wa, Katsute no Deshi ni wa Zettai ni Baretakunai: Kento Kikan

Blythe, the hero of national salvation, or Sword Saint Blythe. During the war, Blythe led his apprentices to save the country with his unique swordsmanship and reincarnated as the youngest son of King Kilpus with his memories as a swordsman. When he turned ten, he skipped grades to enter the high school of a knighthood school in search of freedom and swordsmanship… It was good until then, but there was a former apprentice, Lili Ituka, who had grown into an adult woman serving as an instructor. A child in appearance and an invincible old man on the inside enjoys both swordsmanship and school life to the fullest! Source: NU