To what extent does the expansion of the pool of test takers, i.e. 50% of H.S. students as opposed to 25%(I'm only guessing) , account for the decline? I know mh kids' high school education has been a lot more rigorous than mine was. Except they don't teach grammar anymore.
Can I ask where you live? The high school textbooks I see today remind me of the textbooks I had in junior high school or even elementary school.