I'm A Middle Schooler Becoming The Demon Lord is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

15 year old demon prince, Myoung Seong Joon. To become the demon lord he has attended a middleschool in the human realm and started the maturity ritual.