Once you take her to your basement, you proceed to, uh, use her. I've done the "Feed/Comfort her" instead of "Torture", so I don't know what the latter does. Dont't forget to feed her or she'll die. As you progress through certain levels of 'intimacy', eventually you'll be given the option to "Set her Free...?".
If you do that (it may only come once per slave), she is returned to her parents with a plausible story; she is now completely under your emotional control. She then returns to school a week later. I've noticed that they're (rather obviously) quite overt, which can damage your reputation. A few weeks later, you'll hear of another student who's gone missing... Rinse/repeat.
However, should you choose the option of 'Free Her' when you first discover her, you'll be a hero, she goes to her family, big rep boost, and then offered a reward - money, or she'll joyfully let you impregnate her. Doing this path ends all slavery options. So if you want a few ex-slaves wandering around (they'll pretty much do anything, anywhere, anytime for/with you), do that a few times before you choose the 'rescue' option.
