Ochiru Seija no Seppun

Chapter 1 "Kiss of the Fallen Saint", Chapter 2 " Kiss of the Fallen Tyrant", and Extra "Confession of the Fallen Saint"

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 (Source : FantasyShrine)

Chapter 3 "Kiss of the Jealous Tyrant" and Chapter 4 "Kiss of the Love Slave" :

Approximatively the same pattern, about Tokiwa's friend, Noshio Atsushi, and Yoshino Yuuto.

Chapter 5 & 6: "Kiss of the Lost Lamb"
Second year student Shizuki hates homos. He's sure that Yamada sensei is one. Shizuki starts to bully Yamada sensei... unaware that Yamada likes to "break in cheeky students." Let the training begin!