We actually do know how to remediate, it's just that schools and school districts don't want to dedicate the resources to make it happen. It is far more cost-effective in the short term just to water things down, inflate grades, and pass kids along.
Here’s an ugly truth about American education:
We literally do not know how to reliably remediate students who fall behind. At best we have theories.
But the reality is that if a kid falls behind in third grade or sixth grade, we are very likely just going to push them through all the way to graduation without ever catching up, or even learning much.