It isn’t hyperbole.

You can fail math all year long, but achieve the bare minimum in the subject during your annual standardized test and pass to the next grade.

Course credits/grades do not ~~affect~~ limit? progression in many/most? US school districts before high school.

I know of districts using some degree of social promotion, but I’ve never heard of one promoting on performance but using standardized tests alone instead of class grades or clearing a certain bar for both grades and standardized test as the performance criteria.

Not social promotion, explicitly illegal in my state at least.

School year F -> STAR test minimum + intervention or summer school D -> graduates

Was that the case in the 1960s/1970s because that was when the author was in elementary through high school? I graduated high school in 1995 and what you are describing was not the case at my school nor any other school that I knew of at that time.

>It isn’t hyperbole.

At best you present a potential way that it might not be hyperbole.

Social promotion. https://en.wikipedia.org/wiki/Social_promotion

Obviously, we don't know the author's actual experience but you think that it is more likely that she failed math and science continuously from elementary school through to high school and just kept getting socially promoted OR it is more likely that the author employed a bit of hyperbole and maybe was just generally a poor student in math and science throughout her childhood?

D's get degrees

In my parlance a D is not flunking though. Flunking means you don't get credit for the class. But maybe not where she is from?