There could be other factors involved as well, such as school -
family incompatibility, multiple
intelligences (where certain children learn best within
environments that aren't offered
in either public or private schools), as well as religious convictions and beliefs that aren't welcome
in the public school system (creationism, for example).
First, even if variation
in intelligence is taken to be largely genetically determined — which they argue is increasingly disputable — it still remains the case that children's
family environments and resources are major factors
in how well they do academically at school.