Affluent students have major advantages when it comes to K-12 education: Among them, better teachers, more access to advanced courses, resources for counselors and a variety of extracurricular activities, which when combined can lead to higher high school graduation and college-going rates than their poorer peers.

Now those wealthy students can add to that list another advantage their less affluent peers don't receive: grade inflation.

A new study on grade inflation published Wednesday shows that schools attended by more affluent students saw less rigorous grading than schools attended by less affluent ones. While median grade point averages increased in both school types between 2005 and 2016, it increased more in the more affluent schools.

"In other words, it's gotten easier to get a good grade in more affluent schools, but not in less affluent ones," says Seth Gershensen, associate professor at American University who conducted the research and authored the report. "The GPA Gap has widened."

Gershensen used statewide data on all public school students in North Carolina who took Algebra 1 between 2005 and 2016 and compared their grades to scores on the state's end-of-course standardized exam. He also compared their cumulative GPAs to ACT college entrance exams.

Researchers have long documented the mismatch between school grades and their performance on tests, but prior research has been limited to smaller pools of students – those who took the SAT, for example, or those at a specific school. Looking at all public school students in the Tarheel State allowed Gershensen to draw conclusions about the difference in grade inflation between poor and rich students, which until now hadn't been done.

The study showed that the likelihood of receiving an A remained about constant between 2005 and 2016 among students attending the same school and who scored similarly on end-of-year exams. But more and less affluent schools experienced very different trends in that likelihood during the same time period: Beginning in 2010, the probability of receiving an A in more affluent schools increased significantly, while beginning in 2013 the probability of receiving an A in less affluent schools decreased significantly.

An analysis of the ACT scores also shows that grade inflation accelerated from about 2011 onward, mostly in schools serving advantaged students.

"I wasn't expecting to see that, and if anything, you might assume to see the opposite," Gershensen says, explaining that instances like the recent graduation scandal in the District of Columbia, in which administrators fudged attendance data in order to graduate more students, had him expecting to perhaps see the opposite effect, if any.

So who's to blame? Gershensen says he expects the culprits are pushy parents and insistent students.

"Both parents and students from more well-off backgrounds have the social capital and confidence to confront the teachers in the first place," he says. "The classic helicopter parent stereotype. If you think about why parents would be doing that, a lot of them are well aware of the high-stakes and potential payoff of going to an elite university."

Such GPA gaps, as Gershensen describes them, can have a devastating impact in driving larger education and socioeconomic gaps.

"When students in more affluent schools systematically receive more optimistic evaluations of their current and future performance than their more disadvantaged peers, they will act on this misleading information," he writes in the report. "That means, among other things, that they will apply to and attend more selective postsecondary institutions. In this way, inflated grades trigger a self-fulfilling prophecy that perpetuates – even exacerbates – existing socioeconomic gaps in educational access and success."

The study looked at the issue of grade inflation more broadly, also finding that many students who receive good grades do not demonstrate mastery on end-of-year exams, and that some students with good grades fail to demonstrate simple proficiency.

In fact, among students with top grades, just 3 percent of students earning a B and 21 percent of students earning an A reach the highest level of achievement. And for those who earned a B, more than one-third, or 36 percent, of them did not even score proficient.

The report underscores that grade inflation can be a double-blow to poor students, both because it can widen the socioeconomic gap when the grades of wealthy students are inflated, and also because when poor students' grades are inflated it may cause them to miss out on tutoring services that could help them catch up, or worse, lead them to graduate high school mistakenly thinking they have the necessary knowledge or skills for college or a career.

One potential fix, Gershensen posited, is designing a system akin to a GDP deflator, which economists use to predict dollar amounts over time. If we knew which schools were more prone to grade inflation, he says, a GPA deflator of some sort would allow a better apples to apples comparison.