Magnus Carlsen is the newest world chess champion, after beating long time champion Viswanathan Anand. At 22, Carlsen is probably the youngest champion of all time. Steve Hsu has a three-year old interview with him in which he says he doesn't know his IQ and doesn't want to. He thinks English grandmaster John Nunn never became world champion because he was too smart - and filled his head with all sorts of useless algebraic topology stuff.
SPIEGEL: Mr Carlsen, what is your IQ?
Carlsen: I have no idea. I wouldn’t want to know it anyway. It might turn out to be a nasty surprise.
SPIEGEL: Why? You are 19 years old and ranked the number one chess player in the world. You must be incredibly clever.
Carlsen: And that’s precisely what would be terrible. Of course it is important for a chess player to be able to concentrate well, but being too intelligent can also be a burden. It can get in your way. I am convinced that the reason the Englishman John Nunn never became world champion is that he is too clever for that.
SPIEGEL: How that?
Carlsen: At the age of 15, Nunn started studying mathematics in Oxford; he was the youngest student in the last 500 years, and at 23 he did a PhD in algebraic topology. He has so incredibly much in his head. Simply too much. His enormous powers of understanding and his constant thirst for knowledge distracted him from chess.
Apparently former world champion Gary Kasparov was carefully tested and found to have an IQ of 135 - about 2.3 standard deviations above the norm - well above average but probably a bit below the "smartest" kid in your high school class.