As she was describing this, I realized that it was most likely static electricity that caused the piece of paper to stick to the plastic ruler. So, I decided to try it myself. Using a plastic comb and a small piece of paper, I charged the comb by rubbing it vigorously against my shirt. As a result, when I brought it near the neutral piece of paper, it attracted the paper in an attempt to neutralize itself.
I thought it was funny that I thought of static electricity when my sister described her teacher's "magic trick."