Tensei shite Shota Ouji ni Natta Kensei wa, Katsute no Deshi ni wa Zettai ni Baretakunai

Sword Saint Brise saved the country with his unique swordsmanship. At the end of his magnificent life, he is reincarnated as the country's youngest prince, still retaining his memories. He seeks freedom again and aspires to enter the elementary school of knight school. Even though he is an invincible swordsman, he unexpectedly fails! "I came to get you." He looks like a child, but inside he's an invincible former swordsman. In his reincarnation, he is picked up by his former disciple?! (Source: Ichijinsha, translated)