Our system of "public education" is at least partially successful at serving its primary purpose, which is to provide free babysitting for children and keep teenagers from prowling the streets during daylight hours. Now, some high-minded reformers threaten to clog up the entire setup by mandating learning.
It's long been understood that passing a grade in school is only lightly correlated with actual mastery of the educational material that is taught in said grade. Mostly we give the slower kids a gentleman's "C" for effort, or—in extreme cases—require them to repeat a grade once, before moving them on up in order to ensure that they don't grow so large that they become a menace to their younger, smarter classmates. Everything is by necessity geared towards the maximization of the mediocre. The system "works," as much as we want it to work. Real learning is a happy side effect, promoted and hoped for but never integral to functioning of the mechanism as a whole.
And then one day you read a story about how some wild-eyed utopian LUNATICS are testing a school system in which—this is not a fictional tale, but rather a real live thing operating in these United States of America—kids are not promoted to the next grade ("Content Level") until (and I assure you that what follows is a direct quote from a respected news publication) "after they master the material—not just because they have spent a year in a certain class."
Has our world gone mad?
Now, teachers must track every student during biweekly data sessions and move them in and out of groups or classrooms based on progress. Students must pass exams to prove they have met learning targets. Students who fail the tests repeatedly aren't moved ahead under "social promotion" but must master the material.
I ask you, reasonable citizens, to imagine just two of the consequences of such a system were it to be installed nationwide: high school graduates with demonstrable literacy, and 29-year-old high school students, who are probably awesome at football.
Not so bad really.