Gakuen Sodom ~Kyoushitsu no Mesu Dorei-tachi~

You're a college student living with your stepsister Miku and are nearing the end of your training period as a teacher. Woken up by Miku as usual, the two of you eat your breakfast, watch the morning news reporting on the escape of a death-row inmate from a local prison, and head off to school together.

Today is your last day as a trainee teacher at St. Marianne's Academy, and during your final class, an armed man breaks into the classroom, taking the entire class—Miku and the teacher included—hostage. Sure enough, he's the escapee from this morning's news, and a serial rapist/murderer at that. Starved of sex, he's out to satisfy his carnal desires one last time with you as his unwilling accomplice and his shotgun as judge, jury, and executioner.

[Based on Wikipedia]