I don't think we have a recipe that says, "Take a child of two non-college educated parents, add primary education ingredient X, bake, and out comes a college-capable high school graduate." The mystery ingredient X has yet to be discovered.
Predictably, lots of commenters showed up and said the missing ingredient X is IQ. What they mean is that students continue to need a high IQ to succeed in today's education environment because, by and large, the prevalent educational techniques fail to simplify the complex concepts students need to learn to be "college-capable high school students." The result is that education remains largely inaccessible to those having low IQs.
I'd say mystery ingredient X will most likely involve something that simplifies the complex concepts that need to be learned, but currently aren't being learned by low-IQ students. (This also extends to low-SES students which largely overlap the low-IQ group.) DI provides a large portion of this mystery ingedient X at the elementary school level. In fact, this is speifically the goal of DI's design:
The net result of meeting these criteria is that DI materials appear to be easy. Possibly the most difficult concept for observers of DI programs to understand is that although the programs seem simple, they meet multiple design criteria that make them simple. The superficial impression of a program done right is that the authors may not understand some of the complexities of the content. The complexities, however, have been addressed and have been reduced to non-complexities that do not sacrifice the integrity of what is taught earlier or what is to be taught later. If the criteria are met, the prediction is that the student will generalize to a specified set of examples including those that have not been taught.
Rubric for Identifying Authentic DI Programs, pp. 18-19.