That means the kids are at least a year behind while getting their FAPE. Most kids that are a year behind have trouble catching up; they often fall farther behind. At that point you have a choice: Do you keep the curriculum at their level and try to catch up or keep on going?
Both have risks. If you fall behind, there's no guarantee you won't need to get farther and farther behind. After all, the kids fell below grade level at some point, and whatever is dragging them down (or slowing them up) may still be at work. But if you do that, what do you do with the inevitable GT kids, the 5% that are at grade level. Too often "school reform" doesn't include the top tier of kids because in an egalitarian society we have to lift up the bottom ... and if we let the top kids not succeed quite as much, equality is easier to attain. In some states that's a large portion of the closing of the achievement gap--the bottom 20% has continued to improve at the same rate they have for 20 years, but the upper 20%'s improvement's slowed.
Anyway, the public school system isn't working for these kids. They've tried fixing them. It's still not working. These are going to be rural kids and minority kids (and if we're talking black belt, it's going to be both).
Some charter schools do well. Others are truly abysmal. Many are just average. If nothing else, they often provide diversity in an otherwise unimaginative or homogeneous school system. If we can't fix the problem, we might as well pat ourselves on the back for rearranging the deck chairs. Over. And over.