I can't see why anyone would have a problem with kids moving on more quickly if that is what the kids want to do.
Unfortunately, in leftist domains there is a strong desire not to challenge the children or let them take on too much. This is especially so with minority children. My story:
I originally attended a bad inner city high school. Credits in the district were awarded based on hours spent in the classroom, and NOT demonstrated knowledge. So my desire to advance without wasting my time on familiar subjects was unfulfilled. There were no choices, no options. My English teacher told me I could probably teach the class myself, yet, I had to sit there, bored out of my mind, watching other students misbehave and learning nothing. It was like glorified daycare.
In the ninth grade they forced me to take Algebra I. Not only did I know the subject very well, I had already taken this class in the eighth grade. But that didn't count. I made my case to the counsellor to allow me to at least take Algebra II. But the "well-meaning" lackey lady worried that I wouldn't be able to handle it, as I was a poor black child. I fought it to little avail. I took my appeal to the head of the math department and we struck a secret deal two months before the end of the school year. I'd have to teach myself Algebra II at home, and if I passed final exams, he'd pull strings. I didn't know about formal homeschooling at the time so I just took a textbook home, taught myself as best I could, passed final exams and received credit. Even though I had spent the year in an Algebra I class and passed those exams as well, they didn't give me credit for that.
Left-wing bureaucracies care more about feelings than achievement.
Because it challenges the liberal's sacred cow of equality. If some kids can do it faster, that means they're smarter or more ambitious...and the class clown might feel stupid as a result - which we just couldn't have /sarcasm
Follow the money...
As a successful "high-school dropout"--I took the GED at 17 and enrolled in college--I was appointed to a committee that had the mission to discover the causes of the high dropout rates in our city.
What a shock it was for me to learn that dropping out of High School to begin college "costs the school district federal funds" and "lowers the graduation rate" which "reflects poorly on the district". So the committee voted 9-to-1 to recommend raising the legal dropout age to 18. They have not succeeded in persuading state legislators, yet, but in 1998, the dropout definition was changed to count "students who leave high school without graduating to enroll in Community College or a GED program."
In NC, teens can take the GED when they are 16 years old. There are two ugly caveats: (1) The state law requires the student to drop out of High School officially, prior to registering to take the exam (link); and (2) Officially withdrawing also means losing the legal right to drive. If teens under the age of 18, they cannot drive unless they have a diploma or are enrolled in High School.
Some resources on success rates, etc of GED candidates are available at this link.
Specifically, vpintheak, these ridiculous restrictions and laws have been put in place to keep public school enrollment levels overly inflated. Warm bodies mean cash, jobs and pensions. Furthermore, there is a genuine concern among well-meaning liberals that kids must have a minimal level of education in liberalism before entering the working world and/or they hold a genuine belief that public schools provide good educational experiences to any child who wants to learn.