Wise Man’s Grandchild

A powerful wizard and sage comes upon a baby whom he adopts and names Shin. The baby is actually a man from our world reincarnated into the sage's world. The sage raises Shin, separated from the rest of society, and teaches him in the ways of magic. It is only when Shin turns 15 years old and sets out on his own journey that the sage remembers that he forgot to teach Shin the common sense and ways of this particular world. And so, Shin goes the capital of the Alsheid Kingdom to enroll in the capital's magic academy. <em class=de-emphasized>(from manga)</em>

Synopsis from Anime News Network