Welcome to Demon School! Iruma-Kun 15 (manga)

Iruma Suzuki has always been eager to please, even at the cost of his well-being. Worst yet, he's the son of two selfish parents who end up selling him to a demon. Thanks to their totally irresponsible actions, Iruma has found himself living in the Netherworld, where he must live and attend school as the grandson of an older demon. Luckily, his new, doting grandfather is there to help, but Iruma will have to figure out how to blend in with his demonic classmates or risk getting eaten. All he needs to do is subjugate rival classmates, summon familiars, and do other typical demon things while never revealing that he's human… Piece of cake, right?