When I was in school (long ago), I always kept to myself, never bothered anyone, but people still picked on me sometimes. Once I was sitting in class and another kid put me in a headlock, for no particular reason. He probably expected me to do nothing. I didn't want to hurt him or start a commotion, but I punched right at his nose, and pulled my punch right before making contact. Well, this must have startled him a great deal, because he jumped back while letting out a loud fart. It reeked. The teacher came into the classroom and she had to clear everyone out because it smelled so bad. But anyways, I don't think he ever messed with me again.
So basically, see if you can make them fart in front of everyone.