GPA is already scaled based on income because poor schools give higher GPAs for the same level of knowledge and skill. The study is useless.
The material may be easier in a poor school and an average student can seem amazing compared to a below average student.
The rich dumb, poor smart both come in from behind the eightball. Rich can buy there way out and poor can work there way in. If I'm mit I want rich smart.. seems less risky.