Home schoolers will tell you they knew this (and results bear them out) found that the effect on a child's educational attainment was, in order;

1) parents' effort is more important than the school effort
2) school effort is my important than the child's effort

Yes, they say parental involvement is more important than even how hard the child is inclined to work.

But the teacher's union shouldn't feel too put out.   The study in Review of Economics and Statistics also found that the socio-economic background of the child's family also affected the child's educational attainment – oops, but that also affected the school's effort toward the child.