This is actually by Sakiya Haruhi and titled Ashita-no-Kimi-wa-Koko-ni-Inai, or The You of Tomorrow is Not Here
Yaoi manga with really cute drawings. Both the Sensei and the student look super boyish. Here is the summary from Mangapark:
"After failing Japanaese history, Mio was forced to go to extra cram classes. Finding out that his Japanese history teacher, Fumichika, was quite understanding and kind, Mio couldn't help but feel an attraction towards him. After he confessed, Fumichika tells him that his feelings won't last, but if he can accept the risk of not having his feelings reciprocated and the fact that he may end up hurt, they can start a physical relationship. But when Mio desires more..."