Not only this but most of the "students falling behind" that you hear about turns out to be about poverty, not about teachers or schools failing the kids. If a child lives in poverty, they are worried about when they'll eat next, are afraid that today might be the day they lose their home, might be scared for their safety in their neighborhood, etc. All of those worries/concerns/fears make it hard to focus on what your teacher is trying to teach you. It also makes it seem irrelevant. If your big concern is whether you'll get to eat dinner tonight or whether this will be the fifth night in a row that you go to bed hungry, figuring out the area of a circle can seem completely useless. Yes, learning pays off long-term, but there are big short-term concerns that drown that out.
Unfortunately, a lot of rich politicians/businessmen who have never had these worries/concerns like to place all of the blame on public schools and public school teachers and then lobby to pull more money from them to fund other schools for them to send their kids to. Meanwhile, the poor kids do even worse, but at least the rich folks have a nice scapegoat.