Ouritsu Mahou Gakuen no Saikasei: Slum Agari no Saikyou Mahoushi, Kizoku darake no Gakuen de Musou suru The Irregular of the Royal Academy of Magic

Synopsis

"Magic." A sacred mystery usable only by those chosen. One day, a child from the slums named Ars finds he has the power to use magic. A sorcerer by the name of Genos Wilzard learns of his talent and raises him, turning him into the greatest of assassins. One day, Genos gives him an order: To enroll at the Magic Academy...!? (Source: Shueisha, translated)