Perhaps this is a total swerve from the point of this thread, but I often worry about kids who "do magic at school all the time". Does magic define you? Are you "the magic guy" or are you just you? I unfortunately know someone locally that is constantly doing magic at their school, it defines them as a person and I feel this is a horrible, horrible thing. Magic should be what you do, not who you are. As Ben said, perhaps only do magic once or twice a week. Let them know YOU as well as the magic.
Okay, end of mini-rant and to the question.
When this happens, I just flat out ignore it. I don't address it, I don't make a funny remark that brings attention to it, I just leave it alone. YOU are the performer, YOU control the situation and what they see. I'll be damned if someone is going to tell me what to perform. If they ask me to do something I just politely say "I'll show you something else". Situation is controlled, they have a good time and no attention is brought to the fact that you left your magnet at home
C