IQ tests are not supposed to measure what you know or learned in school, but rather your native innate intelligence. Studies of identical twins reared apart suggest that by late adulthood IQ is almost entirely genetic. There are some environmental effects (i.e. nutrition) but cultural factors like being raised in a rich educated home are now believed to have zero impact on IQ by late adulthood. There are some psychologists who argue that dropping out of school causes your IQ to drop dramatically, however one could argue that people may drop out of school because they were already genetically getting dumber (it just takes a few years for one's IQ scores to catch up with the current state of one's intelligence).
Some people complain that IQ tests only measure a narrow aspect of intelligence, like book smarts, and ignore things like social intelligence, musical intelligence, however propopents of IQ tests argue that IQ tests reduce intelligence to its bare bone basics: Logic, insight, speed, and understanding, cognitive flexibility. If you have these basic abilities you can adjust your thinking to almost any new problem. Further, IQ test proponents argue that they are just taking a sample of one's intelligence by measuring the broadest most general abilities, and it would be impossible to test all the millions of mental talents humans display.
Controversial books like the Bell Curve argue that IQ is the single most important variable in success, and one of the authors writes that even among siblings who grow up in the same family, the high IQ sibling tends to make more money (the low IQ sibling is more likely to end up in jail).
And although conservatives love IQ tests because they justify capitalism and social inequality, conservatives seem to score a bit lower on them, at least social conservatives, religous people, non-altruists, the non-compassionate and those with racial prejudice.
A few IQ scores of famous people: Muhammad Ali 78, JD Saliger 104, Charles Manson 109, Jon Gotti 110, John F. Kennedy 119, Al Gore 135, Bill Gates 170
Average IQ is 100. Only 1 in 100 Americans has an IQ above 135, and only 1 in 1000 Americans score above 146, and only 1 in a million score above 170. It should also be noted that IQ scores increased by 3 points per decade during the 20th century, so in terms of raw test performance, an IQ in of 100 in 1930 is equal to an IQ of 76 in 1990. This phenomenon, known as the Flynn Effect, is causing major problems for the death penalty where laws state that no one with an IQ below 70 can be executed (those who take newer tests are more likely to be executed because rising norms make it harder to score above 70). Some argue that society could not possibly have become that much more intelligent in such a short span of time, so the Flynn Effect proves that IQ tests do not measure real intelligence, but just narrow test taking skills. Others argue that if the average American man could grow three or four inchest over the 20th century, there's no reason why nutrition did not also make him a few dozen IQ points smarter. They point to our technological explosion as evidence of a smarter society. Certainly the fact that Obama is winning the youth vote by huge margins seems like evidence we are getting smarter.