Watashi no Dainiji Seichou

Literature teacher Makoto Kosaka's gentle personality and voluptuous body has his male students comparing him to the a holy saint. Although he is self-conscious about his large physique, he loves his students and his job, and enjoys the kind attention of the other faculty members, particularly that of Shun Hayami, the fit and cheerful gym teacher. One afternoon, after a long and tiring workday, Makoto finds himself overcome by a sudden urge. With the school seemingly empty, he figures no one will mind if he gives himself some quick relief. That is, until Shun shows up out of nowhere...

(Source: Renta!)