I have used a similar tactic to PennyAM. Many moons ago, it was the end of term and an unruly year 8 class, who for the most part I got on OK with. One lad was being a bit of know it all, and I said the 'if you can do it better, come on then' line. He accepted my offer, and went to the front of the classroom. It was great, he suddently realised what hard work it could be when people are not listening, especially as I engaged in some of the low level behaviour that he was regularly part of. (Talking, asking for a pen, looking for the mobile, asking for books, talking again, interrupting him....) Lets just say it was a very worthy 5 minutes or so, he gave up and the class, although not perfect, were a great deal better afterwards!