The trouble with preventing teachers from allocating the credentials themselves is that it makes it impossible for the teacher to grade based on performance in class.
"Who cares," you might ask, "so long as the student passes the test(s)?"
The trouble with THIS attitude is that it promotes cramming, as students do no work during the year, cram to pass the test, and then forget the material. Providing incentives to work throughout the year is a valuable part of the educational process.
There's still room for a private credential organization, but ideally it'd be one that gave tests once a month, or ideally bi-weekly, on a progression of material, up until the final. However, once you're talking about bi-weekly standardized exams, the administrative costs may begin to increase rapidly.
-unless you love someone-
-nothing else makes any sense-