Breeding Classroom: Suspicious Chemistry Lab (2004)

Sachiko Uchida skips class and rests in the infirmary. That day, she sees a couple entangled in the bed next to hers. It's Narumi, a classmate, and Adachi, an upperclassman. Seeing the two begin having sex in hushed tones, Sachiko hastily leaves the infirmary. Then, one day, Yuki jumps from the school roof. She had been made a sex slave by a certain teacher.