Perhaps the most important reason that higher education often fails to live up to its promise is non-completion. More than a third of students don’t earn a degree within eight years of starting college, and only a minority finish on time. College dropouts face lower earnings and more difficulty paying down their debts, among other problems.

Analysts have devoted plenty of ink to the problem of college non-completion, why it persists, and how to turn things around. But it’s impossible to ignore one of the most fundamental factors behind non-completion: students who are less prepared for college are less likely to finish.

That’s one conclusion of a new report from the National Center for Education Statistics (NCES), which tracked a group of high schoolers through college and beyond. Roughly three-quarters of those students enrolled in postsecondary education, and among those 40 percent failed to earn a degree or certificate within eight years.

But different groups of students have vastly different completion rates, and no factor is as important as the student’s high school GPA. Among students with an “A” average (GPA of 3.5 or higher), nearly 90 percent finished college. But among students with a “C” average or worse (GPA of 2.5 or lower), two-thirds of those who started college failed to earn a degree or certificate at all.

To be sure, other student characteristics correlate with high school GPA, so it’s possible some of the gap is due to these other factors. But the gap in college completion rates between students with the highest and lowest GPAs is far greater than the gaps along racial or socioeconomic dimensions. For college completion, high school GPA seems to matter more.

The reasons are obvious. College-level coursework can be rigorous. It demands a sufficient knowledge base and appropriate study habits. High school GPA, to an extent, is a signal for those characteristics. It demonstrates not just a student’s capacity to learn the material, but a willingness to show up to class and complete assignments on time. Those habits are indispensable in higher education.

Unfortunately, high school GPA’s utility as a signal is eroding. As former Institute of Education Sciences director Mark Schneider has pointed out, the average high schooler’s GPA is climbing. Yet more objective assessments of students’ knowledge show flat or declining trends. In other words, students are receiving higher grades, but they aren’t learning more.

Grade inflation is happening at the college level, too. College completion rates have ticked-up a few percentage points over the past two decades, but this is mostly down to easier grading, according to a recent analysis. If students complete college at higher rates, but don’t actually learn any more, that somewhat defeats the purpose of higher education as an avenue to a more knowledgeable workforce and a stronger economy.

The lesson should be different. First, policymakers ought to understand that K-12 education is critical to college completion. If the goal is to raise college graduation rates, we must ensure that students possess an adequate knowledge base and good study habits before they ever set foot on a college campus. Neglecting the K-12 side and trying to make up for it in the college years is usually too late.

Second, we should recognize that college shouldn’t be the only path for students exiting high school, or even the primary path. Even accounting for improvements in K-12 education, it’s fine to acknowledge that the traditional academic setting isn’t everyone’s forte. But that means policymakers ought to work on expanding non-college pathways into the labor force so people who excel outside the world of academia can still enjoy a good middle-class career.

College noncompletion continues to be a major problem, and it’s most acute for students with the lowest high school GPAs. But perhaps it’s time to look for solutions outside of higher education.