USA Today reports "A's on the rise in U.S. report cards, but SAT scores founder" (17 July 2017) — and I think, "Here we go again."
Fairly frequently over 35 years I handled grade complaints in a good-size university department of English. I usually mentioned in my annual reports that there were fewer complaints than we should have in a department as large as ours, so it was likely that colleagues were grading too leniently.
I believed in the existence of grade inflation and that grade inflation could hurt returning students whose, averages had been good back in the day but looked less good against the inflated grades of younger competitors. Still, grade inflation was balanced by requirement inflation so there was some fairness: "Back in the day," one could graduate college with 120 semester credit-hours; by the time I left teaching, the more usual requirement was 128 hours: a half-semester more.
Measuring high school GPA's against SAT scores must be done carefully, however, and reported carefully. For one thing, you need the numbers for who's taking the tests.
In the 1970's, Richard Ohmann examined the "literacy crisis" and reported on it in an essay in The Chronicle of Higher Education (Oct. 25, 1976). "Johnny can't read / Johnny can't write," and the cause had to be radical changes in American high schools in the 1960s.
What had changed most was more young women going to college, so the issue was good news with Jane, not degeneration for Johnny: college teachers were seeing a more typical sample of (female) high school graduates.
There has been grade inflation, but SAT averages are affected strongly by who's taking the tests, and we should expect a decline in scores because of the mostly good news that more high school grads are going to college meaning more people are taking SATs meaning a more typical sample of US high school students — and lower scores.
Ohmann, Richard. "The Literacy Crisis Is a Fiction, if Not a Hoax." The Chronicle of Higher Education 25 October 1976.
Erlich, Richard D. "[…] The Parable of the Masturbating Madmen." Views from a Jagged Orbit. Rockville, MD: Wildside Press, 2017: pp. 141-43.