From Chibi Manga:
Hibari, a high school student, is actually going out with the biology teacher, Aoi-sensei. A love between a teacher and student must be kept a secret! Mail, phone, dates, kisses…anything lover-like are all banned! It’s a relationship where they will break up if there’s even a rumor about them. Because of that, the feelings of love must be protected…But Sensei, aren’t you kind of red? It is a pure and restricted love with the serious biology teacher, the love of a heart throbbing every day will begin!