Sensei, Machigattemasu.

There's a new male teacher at a longstanding all-girls middle and high school! His name is Karato Kentarou, a young man just out of college who has been hired on as a middle school English language tutor. However, he has a big secret. He's not really in his twenties or qualified to be a teacher; he's a 16-year-old boy who missed his high school entrance exams due to unfortunate circumstances, and ended up in this job. He's happy to be surrounded by hot girls near his age who look up to him as a teacher, but things could go very badly if his secret gets out!

(Source: MangaHelpers)