If you have to hide the fact that your messing with her, you are not worthy of lessons!
Begone from my sight coward!
When your feeling courageous: get a toilet roll and a bottle of water, remove the carton from the middle and douse (sp?) the toiletpaper in water so it gets soggy like hell, then the second she turns away from the class fling it at the black/whiteboard right next to her.
Then just laugh your ass off with the rest of the class before she removes you from the room!
Saw this in a movie, did it myself.
If your gonna be childish: make the most of it!