Can you elaborate a bit more for those who are not part of the US but who might offer some insights from a different perspective?
You mention parents twice..
Evolution has nothing to do with it btw. If it had, you should be able to conclude that from the numbers.
Last things first .... the data clearly demonstrates teaching evolution has nothing to do with the decline of education in the US. If it did, the top ten or so nations wouldn't be teaching evolution. When we step back and look at the big picture, there has been a lot more going on in society during the post WWII era than just the teaching of evolution.
When I was composing the list, the lack of parental involvement in their children's education was the first thing I thought of. Later, I thought of some other factors like two-income households, latch-key children, and other failures to put the welfare and education of their children first, so I just mentioned them again.
Now, by way of background, elementary schools are grades 1-8 (age 7-14 at the completion of the year). Secondary schools are called "high school" and are grades 9-12. Education is pretty much guaranteed thru grade 12 (about age 18). After that, if one continues their education they go to "college." As I understand it, Europeans call their secondary education "college" and post-secondary "university." Over here, the terms "college" and "university" are interchangeable.... but I try to use "university" to avoid any confusion.
Under our Constitution, the 50 individual states are responsible for education. The central government has no official authority but they use the "power of the purse" to .... shall we say "encourage" the states to comply with federal mandates.
The decline of academic performance of US students probably began around 40-45 years ago. One lingering effort to reverse the trend was the passage of the "No Child Left Behind" (NCLB) law about 15 years ago. Since then, there has been a big effort to hold schools "accountable" for the decline in the performance of our students. This has led to an increased emphasis on standardized tests. Teachers are told we should not teach to the test, but we will be held accountable for how well our students do on these test.
NCLB also created new and expensive mandates for schools to provide additional "special education" programs. As a result, 10-20% of students get special accommodations such as a reduced number of choices on multiple choice exams, word banks, calculators, reduced numbers of questions, lower passing scores, extra time and more. Many of these students have figured out the game and simply work the system.
Some states tried to address the poor performers who had not met minimal academic standards by giving them a "Certificate of Attendance" rather than a diploma at 12th grade graduation. They were taken to court under NCLB and lost. As a result, we grant an 18 year old who can't multiply 5x5 the exact same diploma as the top students.
To put it bluntly ..... a high school diploma is meaningless.