My DD knew the results of her IQ test as she opened the letter, it was addressed to her. We had her tested as she was always told by the teachers at the end of the school year that she was really bright but as she was now moving up there was nothing they could do about it.
She told no-one until one rather annoying "bright" boy was bragging he was so close to joining Mensa, but didn't quite make it, and he was obviously the brightest in the school. My DD turned to him and told him that Mensa was not that great and she had left it, apparently the look on his face was priceless. (she was 16 at this point)
I think we do our kids a mis justice sometimes and they can sort themselves out