I already posted a cynical explanation of why they do this. But suppose they are acting in good faith and are reluctant to give credit because they want ensure that students properly learn the material. Is there documentation they will accept, for example a student notebook with completed assignments, supplemented by a proctored exam?
Nope. I tried that, both with his public school and DD's private school. They refuse to even consider anything that doesn't have an
accredited stamp on it.
I ran I into a similar mindset problem when DS13 had skipped third grade. He wanted to do fifth grade science, and it wasn't allowed because the person running the school had decided that you couldn't possibly understand fifth grade science unless you had done everything in fourth grade science. The books all basically told the same story, but each grade level gave more detail and used more technical words. He just wanted to learn more details, but it was too bad.
Unfortunately, our education system in general just don't get it on many levels.