Ochikobore Yuusha to Rikka no Majo

Shuu, an underperforming student at the Hero Academy whose swordsmanship and magic skills have not developed, is finally advised by the headmaster to leave the academy! Two beautiful witches suddenly appear before him and reveal he is the chosen one who has inherited the soul of the Demon King, whose mighty power he needs the cooperation of six witches to manifest. Shuu quickly masters the basics of magic, and for the first time in his life, he is able to use magic and successfully clears the final examination on which his expulsion hangs!! Shuu then decides to first become a hero with the power of the Demon King as a prerequisite to becoming the Demon King that the witches desire... The peerless boy Demon King's academy fantasy begins!

(Source: Takeshobo, translated)