Missing 55 by Yasushi Koshizaka

3.33

There is a "school" that abducts and confines girls to re-educate them as sex slaves. Kyoko, a bad schoolgirl, was caught by a witch and turned into student number 55. Life in the school is hell where repeated torture, ill-treatment, and rape is called "re-education". However, Kyoko has a hidden purpose. She was trained by a secret organization called Onesti and infiltrated the school as a spy. Her mission is to retrieve top secret data from the trafficking organization and release the captive girls. Kyoko works with classmates Yuka, Yuma, and Yui to formulate a detailed escape plan. Finally, the day of their jailbreak has come...