Roku Hime wa Kami Goei ni Koi wo Suru: Saikyou no Shugo Kishi, Tensei Shite Mahou Gakuen ni Iku

Vance is a guardian knight who served the Empress Arsmeria of the Celestial Empire, one of the Seven Empires. When Arsmeria sacrificed herself to end a war, Vance performed the "Reincarnation Ritual" and gave up this life so he can be reborn with Arsmeria in the future. A thousand years after the war had ended, Vance was reborn as Lloyd. He was adopted by the family of a count in the Celestial Empire and raised as their own. At the age of fifteen, Lloyd sets out to find his master, who should have been reincarnated at the same time as him. He enrolls in the magic academy where students gather from all over the world, looking for the person he swore to protect. And this time, he will keep her safe to the very end. (Source: Kodansha, translated)