The New York Times article "After the Bell Curve" revisits the issue of genetics and IQ. Reporting on a number of recent research projects that study twins and orphans from different economic backgrounds, it finds that nurture - in the form of mental stimulation and challenge - plays a much larger role in intellectual development than previously understood.
Taken together, these studies show that the issue has changed: it is no longer a matter of whether the environment matters but when and how it matters. And poverty, quite clearly, is an important part of the answer.
That is not to say that an affluent home is necessarily a good home. A family’s social standing is only a proxy for the time and energy needed to keep a youngster mentally engaged, as well as the social capital that helps steer a child to success. There are, of course, many affluent parents who do a bad job of raising their children, and many poor families who nurture their kids with care and intelligence. On average, though, well-off households have the resources needed to provide better settings for the fullest development of a child’s natural abilities.
Cognitive Daily, the blog which linked to the New York Times article, also points out that IQ isn't everything when it comes to school. Self-discipline, not just braininess, is a good predictor of academic success.