Onna Kyoushi Otoka "Naburi Nawa"

Suzunari Otoka is a newly graduated teacher. When she notices the bullying going on in her class, she gets angry and hits the ringleader, Saito. Angered, Saito feigns remorse, and asks Otoka to come to the classroom after school. However, it is a trap to sexually humiliate her.
Otoka is tied up, and forced to perform fellatio. Subsequently she is forced to have anal sex as well as double penetration. Otoka's sexual insult continues through the night until daybreak.

[From DLsite English]