“They sit there and blink. They approach sub-mediocrity,” says a former coworker when I ask her to describe her “D students.” She’s only partially joking.

Getting an F typically requires some combination of compulsive truancy, a keen distaste for holding a pen, and problems outside of school. An F leads to summer school or an online course, and unrepentant F students tend to drop out or head to an alternative school before long. Fs are a serious problem in education.

D students, however, often stick around and cause another serious problem: They may pass, but they learn close to nothing along the way. Plus, they have little chance of attending a four-year college out of high school. A D student may flake on at least one major assignment a semester but breezily make up minor reading quizzes two months after they were originally administered. Maybe he shows up—but only after sauntering in 10 minutes late. Maybe he doesn’t ask for help and casually breaks appointments for tutoring. Rarely reading and occasionally despairing (with a smile) that he “can’t understand the book,” the D student probably falls behind early and catches up late. But not too late to prevent that bad grade from morphing into a worse one—and not wholeheartedly enough to get the C or B he’s likely capable of earning. This D student knows exactly what he needs to do to avoid an F before grades are due.

Fs are rare in my 10th- and 12th-grade public-school literature classes. While I would like for Ds to be rare, too, 18 percent of my students earned one by the end of the spring semester.

Unlike the few who got Fs, they received the same amount of credit on their transcripts as did anyone with an A, B, or C. They just probably won’t be going to a selective college (at least any time soon). In California, where I teach, state universities from Berkeley to Chico State don’t admit any student who got a D in a prerequisite core class, like algebra. And most reputable private colleges across the country set similar expectations.

As educators, politicians, pundits, and parents debate the logic of Common Core testing and deliberate how to best hold teachers accountable, inspire students, and improve educational opportunities for American kids, it seems counterintuitive that what my former coworker calls “sub-mediocrity” can lead to a diploma. The bulk of my 12th-graders passed the California High School Exit Exam as 10th-graders. (Every public-school student in the state must pass the exam in order to receive a diploma.) Still, a healthy minority of those seniors left high school a few weeks ago with abysmal GPAs—largely because of Ds—and will probably soon enter the slow-moving currents of part-time community-college attendance. Community college is a fine continuing-education option for people who are committed to their studies, but too many students let it become the only one by racking up Ds.

Although I dislike reducing student performance to simple letters and numbers, studies show that grades are a better indicator of college success than is test performance. Unfortunately, when students know that Ds will earn a diploma as readily as As will, some game the system. If pride, intellectual curiosity, social pressure, and vigilant parents do not compel them to do otherwise, some students only work to avoid getting Fs. These students appear indifferent toward class discussion and content. Those habits often follow them after they leave high school. In my correspondence with former students who have already graduated, I have observed that the kids who became accustomed to Ds in high school often struggle if and when they’re in college because they never developed the academic and personal skills necessary to succeed there. Without the oversight of teachers, counselors, and parents that they may have had in high school, they are freer to fall—and to lose what scant interest they may have once held.

Eliminating Ds might reduce the behavior that tends to cause them. Some schools and districts in the country have already done so, including a New Jersey school district that banned them in 2010. I once worked at a Los Angeles charter school that did away with Ds to increase college acceptances. At this school, students “failed” a class when they scored below 62.5 percent—a cut-off number derived from a five-point grading scale that was based on state standardized tests. Yet very few scored below that threshold. The percentage distinguishing a C- from the abyss of failure below was significantly smaller than it had been before, but it was the midway point between “Basic” and “Proficient” on rubrics using that five-point scale. At the end of the school’s first year without Ds, over 90 percent of the senior class accepted invitations to attend four-year universities, and most students had at least entertained the option.

The elimination of Ds at the charter school required teachers and counselors to frequently remind struggling students of their precarious status and give them, when needed, extra help before school and at lunch. Tellingly, the strategy worked at a small school with a small student-to-counselor ratio. Few students missed assignments, and many rewrote essays until they were good enough. One could argue that such a system potentially allows subpar students to just barely nudge up their grades, permitting entrance into four-year higher-education institutions from which, with their abilities, they may struggle to graduate on time or at all. So far, though, my former C- students have not stumbled.

Ds may, in an odd twist, benefit schools more than students. While no school I've known likes transcripts brimming with Ds, schools are really scared about the prospect of seniors not graduating; many of them face sanctions if they don’t report high-enough graduation rates. Mass failures also create a logistical nightmare for counselors and administrators who must answer to enraged parents, placate concerned district and board officials, and find kids make-up opportunities.

Schools may not be able to accept the risk of more failing students, if only for a transitional period. Even a year in which the failure rate doubled or tripled would be dangerous. Many schools couldn’t provide the aggressive counselor interventions and tutoring a gradebook without Ds might require. But if Ds are markers of adequacy that everyone recognizes as inadequate, doling them out seems illogical and cynical.

I wouldn’t be opposed to Ds if it weren’t so easy to get one without doing much. Some teachers give kids an automatic 60 percent (i.e., a D-) for turning in an assignment with nothing more than a name at the top—and that’s problematic given that, as reformers stress, students suffer due to low expectations. Other teachers push students to earn every point they receive; in a perfect world, a D would be the lowest grade that a kid gets when she enthusiastically confronts challenging material. The student still learning English while tackling grade-level college-prep content in a mainstream literature class doesn’t deserve to fail. The student in the same class with specific learning needs who requires extra time for assignments and shortened texts doesn’t, either. In my experience though, the majority of these traditionally high-risk students can get a C.

A student is not the sum of his or her high-school transcript. The D students may not end up in prison or in minimum-wage jobs, but they are preordained to face extra obstacles to future goals—goals they may not have yet set. Meanwhile, some students never realize that they have the potential to do more than man a cash register. There’s nothing wrong with deciding against college, but a high-school education should give all students the opportunity to sort through as many options as possible. Allowing them to opt out at the age of 14 shouldn’t be one.