At the core, you are correct. If each student had parents who made sure their child was learning what he needed to know, there wouldn’t be that problem.
But most parents aren’t that aware. They see the report cards and see A’s and B’s and think that little Johnny and Suzie are doing just fine.
Johnny and Suzie don’t feel they are falling behind because they are doing as well as everyone else.
So how is an average parent, who sees A’s and B’s and whose kids don’t think they are falling behind, supposed to realize what a failure their child’s education is?
Parents need to realize that schools nowadays teach to the lowest common denominator. And now with Internet and the vast wealth of resources out there, there is no excuse for kids not to be well ahead of where the class is. Parents need to review the work their kids are doing in class, and not merely rely on grades to gauge their progress. It's not rocket science.