It's easy to see the results, but here's the how and why.
There is the impression that local schools control what is taught to the student, but to be more accurate, they have little say. Even before the advent of Common Core, the tendrils of Federal power extended deeply into the classroom via Title funding for education.
With money comes the power inherent in the ability to withhold those funds.
Under that threat, even the smallest and most remote of school systems will comply with Federal requirements, seeking people to pigeonhole into the various programs for the funding each little head brings in. The body count becomes all-important, at least initially, but in the end, even lunch menus suffer.
Now, the pressure to fall within parameters in what passes for literacy and mathematical proficiency encourages those in secondary education to trim the herd so the statistics fall within desired limits. Students who are not in one of the 'special' programs designed to cover underachievers as well as those who have genuine challenges to learning, either fall within the desired statistical parameters or, if they underperform, for whatever reasons, (in some cases I personally know of) are actively encouraged to drop out.
Yep, you read that right. ...
are actively encouraged to drop out. One such was a transfer student who moved to the closest large town from a larger town, who did well (3.3 GPA) in her old school, but who found that the lessons did not flange up seamlessly between the school she left and her new one. In mathematics, as in other subjects, that disconnect can result in conceptual problems (especially in mathematics), and the student sought help from the faculty--who put her in a room with books and worksheets and left, and when she failed to figure it out on her own, encouraged her to drop out.
Finally, she did, and went on to get her GED before her class graduated.
But passing a test isn't the same as filling in the gaps between the questions, and I question whether the statistical results of education establishments haven't surpassed imparting knowledge and the ability to think in priority. At least she 'missed out' on the social and socialist indoctrination and other nonsense.
High School isn't <I>American Graffiti</I> any more, if it ever was.