Not my intent to pile on here, but this is a pretty fascinating topic.
For what it’s worth, a 2017 College Board study showed that from 1998 to 2016, GPAs rose at a higher rate in private schools than in public schools.
Private Independent Schools: 8% increase
Private Religious Schools: 6.4% increase
Suburban Public Schools: 2.4% increase
Urban Public Schools: 0.6% increase
https://qz.com/1058476/grade-inflation-is-the-worst-at-rich-private-schools-disadvantaging-poor-students/
I haven’t read the study to determine whether they controlled for increased availability of AP courses in private versus public. And some have questioned the source, as College Board would have a vested interest in deemphasizing the predictive value of high school GPAs.
Another study by the Fordham Institute focused solely on a single class, Algebra 1, and whether and to what extent there was grade inflation between 2005 and 2016 among North Carolina students. That study showed that while grades rose at all schools from 2005 to 2016, the rate of increase was 30% greater at more affluent schools versus less affluent schools.
Interestingly, the study also found that starting in 2010, if you control for standardized End of Course (EOC) scores, it was easier for a student to receive an A in more affluent schools compared to less affluent. In other words, for two students who received the same standardized Algebra 1 EOC score, the student at the more affluent school was more likely to receive an A grade in the class than at the less affluent school.
The study can be viewed here:
https://fordhaminstitute.org/sites/default/files/20180919-grade-inflation-high-schools-2005-2016_0.pdf