Could we look at average SAT scores by year as a proxy? It's not a great metric because not all students take the SAT but I think there's some correlation.
https://blog.prepscholar.com/average-sat-scores-over-time
There was a major change to the SAT design right around the start of the period of interest which seems to have caused scores to jump.
There was a major change to the SAT design right around the start of the period of interest which seems to have caused scores to jump.