Yama ni Suterareta Ore, Tokage no Youshi ni naru: Mahou wo Kiwamete Oya wo Koeta Kedo, Oya ga Densetsu no Koryuu datta Nande Shiranai

Hadar was abandoned on a mountain as a sacrifice soon after his birth. However, he was saved by a legendary old dragon and raised by him until the age of 12. Hadar, who had always possessed strong magical powers, was trained by the dragon, and without even realizing it, became the "most powerful creature" who could wield the most powerful spells. One day, to help him adapt to human society, Hadar is told by the dragon that he must attend a school for humans. Coming down from the mountain, Hadar makes full use of his magical powers and becomes a peerless wonder at school! (Source; Lanove, translated)