Nemuru Usagi(2009)

Nemuru Usagi

To impress the girl he has a crush on, Kouichi a high schooler let her use his address in gay add, then the reply is coming, and Kouichi found himself met with the man he made joke of, intend to reject him. But he was suprised to find the man is actually a teacher in his school. Hirokazu is a really nice man, and seem so lonely Kouichi just couldnt find himself to hurt him. one date lead to another, and Kouichi fall for real for his teacher...the problem is, Hirokazu doesn't know that Kouichi is a student at his workplace.

(Source: Astatalk)