It’s not that those students have been getting smarter. Even as their grades were rising, their scores on the SAT college-entrance exam went down, not up. It’s that grade inflation is accelerating in the schools attended by higher-income Americans, who are also much more likely than their lower-income peers to be white, the research, by the College Board, found. This widens their lead in life over students in urban public schools, who are generally racial and ethnic minorities and from families that are far less well-off.

“This is just another systemic disadvantage that we put in front of low-income kids and kids of color,” said Andrew Nichols, the director of higher-education research at The Education Trust, a nonprofit advocacy organization. Nichols was not involved in the research.

The grade-point average of students at private high schools who took the SAT climbed between 1998 and 2016 from 3.25 to 3.51, or almost 8 percent, the College Board found in research to be published early next year.

In suburban public high schools it went from 3.25 to 3.36.

In city public schools, it hardly budged, moving from 3.26 to 3.28.

“If there were a uniform upward drift, then we would have one problem,” said Michael Hurwitz, the senior director at the College Board, who led the research. “But this drift causes another problem: The variation does seem aligned with wealth in a very troubling way.”

These findings are troubling, but not surprising, said Richard Weissbourd, the director of the Human Development and Psychology program at the Harvard Graduate School of Education. “To be attractive to parents,” private schools in particular, Weissbourd said, “need to be able to tout how many of their students went to selective colleges. So they’re incentivized to give better grades.”

The same concern about college admission drives parents of students in suburban schools to pressure principals and teachers, he said. “It becomes very high maintenance for schools to deal with aggressive parents. So that can also push grades up.”

Then the cycle repeats.

“This is one of those things that works like a contagion,” Weissbourd said. “If you’re an independent school or a suburban school and you’re giving Bs and the school in the next community is giving A-minuses, you start to feel like those kids are going to get a leg up. So you start giving out A-minuses.”

Public schools in urban areas seldom seem to feel the same pressure. When Olivia Hall’s mother calls the public high school in Pittsburgh where she’s entering her sophomore year, the 15-year-old said, “They put her on hold and tell her the principal and guidance counselor are busy.”

All of this throws up yet another barrier in front of urban public high-school students, who already face an obstacle course of challenges to getting into college.

The problem takes on even greater consequence as growing numbers of admissions offices make ACT and SAT tests optional and rely still more on GPAs. (As to whether the College Board, which administers the SAT, is acting in its own interest by drawing attention to these trends, Hurwitz said the organization simply has the greatest access to test and grade data.)