Over the long term, average high school grades have gone up while SAT scores have “remained relatively unchanged”.
Average SAT scores going back to 1952, courtesy of Erik Jacobsen, also known as Erik the Red:
The solid circles and triangles represent scores calculated on the recentered scale.
In April 1995, the College Board recentered the score scales for all tests in the SAT Program to reflect the contemporary test-taking population. Recentering reestablished the average score for a study group of 1990 seniors at about 500—the midpoint of the 200-to-800 scale—allowing students, schools, and colleges to more easily interpret their scores in relation to those of a similar group of college-bound seniors.
By 1995, average SAT scores had drifted downward before recentering brought them back up.
The effect of recentering was to raise SAT scores for almost all tests taken before 1995. For example, a 540 verbal score from pre-1995 is equivalent to a 610 by today’s standards. The impact of recentering was smaller for math, with a pre-1995 math score of 540 translating to 560 today. All scores can be converted by using the CollegeBoard table.
(A comprehensive history of the SAT and ACT tests going back to the late 1800s is also provided by Erik the Red.)
What about high school grades? Have they also declined over time? No, in fact they have risen, especially for students with lower standardized test scores.
… Studies from ACT and College Board, the companies that run the two preeminent college-entrance exams, show GPAs increased while scores on the standardized ACT and SAT did not, a phenomenon they say likely indicates inflation. ACT estimates the average GPA inflation was about .25 on a scale of 4.0 between 1991 and 2003, though the 2005 study’s authors believe even that number understates the actual amount of grade inflation. Whether from hard work or grade inflation, GPAs grew the most for students with lower standardized test scores, and the least for those with higher scores on the SAT or ACT.
Another source indicates evidence of high school grade inflation over the long term.
Camara, Kimmel, Scheuneman, and Sawtell (2003) investigated grade inflation by using self-reported grade data and SAT scores for eight cohorts of college-bound students. The trend of average HSGPA over time was examined to investigate grade inflation. The authors found that 2002 high school grade point average “far exceeded” the grades students reported in 1976, whereas the SAT-V and SAT-M scores remained relatively unchanged.