Retired Demon King is a Manga/Manhwa/Manhua in english language, manhwa series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

When the Demon King feels challenged by the hero to find love, he infiltrates the school for human adventurers where romance thrives the most, only to find that the school is under the holy church... which main purpose is to kill the Demon King.