I like the "one in a thousand" sort of contrast as IQ has a some overlap with achievement, but is not a good indicator for an individual.

http://www.iqcomparisonsite.com/iqtable.aspx

Will let you translate an IQ score into a ratio. So you might say, one in ten million people score a 178, that means in a city like New York with a million kids in their public school system they may see a kid like mine once every five or ten years.

You could also use the standard deviation groupings for scale. Like "A kid with a greater than 160 IQ is as different from a typical gifted student as an average student is from a child with a 70 IQ."

Or talk about rates of progress, such as a gifted student could progress through regular material at a rate of 1.5 years per year of school, a highly gifted (above 145) one may progress 2 years each year, and a profoundly gifted student (above 160) could progress 3 or 4 years per year... given the opportunity.