Drops of Desire (2009)

Blurb:
On what should be a routine trip to the nurse’s office, Yuzuru walks in on school nurse, Kokuyo, as he’s ravaging one of Yuzuru’s fellow classmates. Unable to stay away from the dangerous yet charismatic man, Yuzuru finds himself an unwilling resident of the nurse’s office, unable to resist the onslaught of pleasure Kokuyo subjects him to or escape the confines of its walls. Will Yuzuru’s fear be overcome by the fleeting displays of warmth this mysterious man gives him, or will the realization that this man isn’t everything he seems have Yuzuru fighting for escape?

Source: www.anisearch.com/manga/11600