One could write a sizeable book on each variable in a child's life and how it influences their academic performance. However, in my experience there is one factor that stands out above all the others as the most important: parental attitude to education.
The studies into the relationship between family income and academic performance tend to focus their conclusions on the low-income families, however I would like to draw attention to the fact that parental attitude has just as much impact on the children of the wealthiest families.
During the course of my academic career I have taught children from the poorest areas of North East England, through the social spectrum to children of some of the wealthiest families in the world. I now run an international private tutors agency, and so interview parents when determining their requirements for a full-time live-in tutor, and also the children who will be taught. Whether wealthy or disadvantaged, the most influential factor on a child's success at school, or in any kind of education, is the value that the parents place on learning, questioning and discovery, and what they do to pass on those qualities to their children.
Across all social strata, many parents have demanding jobs that reduce the amount of time they can spend with their children. Wealthy families who have this positive attitude towards learning are willing to devote some of their financial resources to providing full-time live-in tutors. This ensures that they are giving their child the best opportunities to learn from highly educated individuals as possible. The fact that the parents hold development of their child's intellect in such high esteem clearly has a huge impact on the child, and their academic performance.
Compare these families to others in the same economic group who do not consider education to be a priority. The parents may employ a chef, a gardener, a driver, all manner of staff dedicated to keeping their lifestyle in perfect order. However, these families haven't made the decision to nurture the education of their child in the same manner. Paying for private schooling is not the same as employing a live-in tutor to truly enhance their child's education, or extend upon subjects taught in the classroom. In these families, parental attitude reduces the educational opportunities for the child, regardless of the financial resources at their disposal. This is why I say that parental attitude is the most important factor in a child's life, amongst the wealthiest families in the world just as much as the poorest, even taking into account the difference between their financial resources.