(Grade inflation is running rampant in US high schools, a new study finds.COD Newsroom/Flickr)

More high school students in the US are graduating than ever before, in part because of rising grade-point averages. But a new study suggests the trend isn't cause for celebration.

Over the last two decades, high school GPAs have risen considerably while SAT scores have fallen. To the researchers behind the study, the conflicting trends indicate schools are engaging in "grade inflation," or the practice of artificially lifting students grades by lowering standards for awarding As.

There are now more students getting As than any other grade, the study found.

Sociologists have observed a rise in grade inflation for nearly two decades, particularly in higher education. In 2001, an education policy committee at Harvard called grade inflation "a serious problem." The problem persists today: The median grade at Harvard, one of the most exclusive and rigorous universities in the world, is an A-. The most common grade is an A.

The new data, presented by Michael Hurwitz of The College Board and Jason Lee of the University of Georgia, reveal the trend has been growing in US high schools since at least the late 1990s.

In 1998, 38.9% of high schoolers had an A average. By 2016, the rate had increased to 47%. Meanwhile, the average SAT score fell from 1026 to 1002 on the 1600-point scale.

Hurwitz called the findings "really stunning," according to USA Today.

When the team analyzed the data, they found grade inflation was most prevalent in wealthier, white schools and private schools. Private schools, in fact, had cases of grade inflation at three times the rate of public schools.

Neither coauthor could explain the trend of grade inflation; the study merely presented data showing it takes place. However, both rejected the idea that students were actually getting smarter over time, based on the finding that SAT scores were dropping.

Hurwitz said the findings reiterated the importance of standardized testing, however tilted the SAT may be toward favoring students from wealthier families.

"We're not saying you should just ignore grades. But what we are saying is that it is important we have some sort of standardized measure like the SAT," he told Inside Higher Ed. "Right now where we see high school grades is enormous variation among high schools and variation of grade inflation."

As a solution, the team recommended a return to class rank, which many schools have abandoned in recent years. Rankings could help admissions officers tell a lot more about a student based on where the rest of the student body falls.