Part of the problem stems from the "everyone wins" attitudes. There have lawsuits over holding students back. Some Child psycho-babilist claim that labeling children as "failures" or making them repeat grades/classes in school cause "psychological damage". Than add on all the "sports stars" that used to get passed through the system but couldn't read or write. Some of this is BS from the Federal Government, some the result of Court decisions.
I won't even go into the whole "equal" education fiasco.
Some states adopted the "exit exam" as a means of identifying those who actually got some education vs stayed in school for 12 years. Some states have also changed it since first being implemented because teachers started teaching how to pass the test, not the subjects.
All in All, it is just one more indicator in the mounds of data about how really screwed up or education systems have become.