Ochiru Seija no Seppun

From Fantasyshrine: Okano-sensei, a high school teacher, was sexually molested by a pervert in the train on the way to school. As fate would have it, his student, Tokiwa, witnessed the whole incident. He then threatened to expose Okano for having a fetish of being groped by a man unless Okano agreed to be Tokiwa’s sex slave! In shame, Okano succumbed to the pleasure only Tokiwa could provide…