Kentei Gakuin no Magan Kenja

"Lag Logline - I grant you the rank of 'sage'," the sorceress Lynne told her young apprentice. But that means he has to say goodbye to his beloved master. In order to save the world a thousand years from now, and to fulfill Lynne's wishes, Lug is sent to the future. However, when he wakes up a thousand years in the future, all he sees is a decaying city and his master's staff. According to Clau, a swordsman girl, there is no one left in this world who can wield magic. By pretending his magic is a form of swordsmanship and the mage's staff a "Holy Sword," Lag enrolls in the Heinel Academy of Swords and Crafts with Clau. However, he is confronted by a girl who looks just like his master Lynne, whom he had left behind a thousand years ago.......?

(Source: Lanove, translated)