ritsu-sama's manga / #Prostitution(3)

Seme is a freshman who love drawing and has a serious face. He was left alone after all the former club members resign due to his intimidating pressure. One day, he met senpai who was alone in the drawing club room. Seme immediately fell in love with his face and requested him to be his model which uke agreed. Seme unconsciously drew a nude uke in a sketchbook which later was found out by uke. Uke misunderstood seme's original intention and offer him to draw himself in nude. Apparently, uke has rumor as being a slut in school. by somebody

According to baka updates it's just it

the one where the uke exposes his ass in a hole in the wall