How to secretly activate camera?

There's this chick in school and she likes to borrow my iPhone 4s duting class. And she like to mess around with it. She also sticks it down her shirt to tease me. I was wondering if I could activate the video camera and get a quick vid of her amazing rack?

There are apps available that allow for stealth camera picture taking. However the apps I have seen just have a black screen that you tap to take pictures without anyone knowing. I would suggest searching for other options in the App Store.