Tsumi to Batsu no Spica

Hatori is a teacher living a typical life at an ordinary high school. One day, he notices that a student of his, Supika Tsumugi, is being bullied. On the way home in the rain, he happens to find himself alone with Tsumugi. Hatori tells her, "I'll do something about it," but Tsumugi holds his hand gently and says with a laugh, "Hatori-sensei is such a liar." A teacher and a student... two people who shouldn't get overly close. A somewhat fascinating girl completely changes Hatori's ordinary life...

(Source: Kodansha, translated)