The Devil in School is a Manga/Manhwa/Manhua in english language, Mature 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

Kasai Koharu, a beautiful girl with an exceptional sense of style, is assigned as the new replacement teacher after the main high school instructor was hospitalized due to relentless bullying from the students.

Though she appears airheaded and constantly wears a bright smile, the students continue to mistreat her, unaware of the dark secret that their teacher, Kasai-sensei, is a real psychopath.

“The reason I became a teacher is to bring happiness to my students.”