It might be true, but I think there's more to it than that. My family has always been lower class, but I'm in the top 5% of my school. I think it's not just the amount of words you hear as a child, but also the quality of the words. As in, the overall level of vocabulary your parents speak in. And of course, parents who encourage literature over television (like mine did) will make a big difference in their child's academics.