The "smartest" (measured by IQ/SAT/whatever your favorite broken measure) of the lowest socioeconomic quintile (lowest 20%) almost never do as well (measured by highest academic attainment, monetary holding, etc.) as the "dumbest" of the highest socioeconomic quintile (highest 20%).
A child can have a really high intelligence, but its usage will be compromised if they're always hungry, always sleepy from being saddled with taking care of siblings or afraid of being abused. A child can have lower intelligence but this can be buttressed by spending lots of extra time and tutors which the upper classes have in abundance.
One of the teachers I knew used to have a wag saying: "When you do wrong when you're poor, you get called thick and can look forward to jail. When you do wrong when you're rich, you get forcibly tutored until you're not an embarassment. When you do wrong when you're middle class, you get called ADHD so everybody can ignore you."