The success of our education system is not just a matter of what schools do. Student outcomes in education are deeply affected by factors outside the school, such as good health care for children and supports for parents that allow them to help their children grow and develop. Access to good jobs with decent pay and benefits for parents, and to adequate housing, are important for children. Environments that respect diversity and support different kinds of students (and families) are also helpful in ensuring that all young people have the possibility of a good outcome from their schooling. So the growing income inequality in Canada, especially when it is linked to such things as immigrant status, is clearly a threat to the quality of our schools.
Simply substitute "U.S." for "Canada" and the story remains the same.