I struggle with this. I think that sure, achievement could not be as high as it would. Certainly you can increase achievement results with direct instruction.
My DD6 was in a gifted K classroom last year. They do a lot of enrichment, but academically, they take a very slow and easy start. She had achievement test done 3 days after starting 1st grade. Her results were very good. It got me by surprise in some areas, like math. Yes, we talk about math when it comes up. For instance, a few weeks ago, she was building triangles with cheerios or some other thing she was eating. Since she was doing triangular numbers, I told her that is what they are called. She then started mentally continuing the list of triangular numbers.
Then, a few days later, for some reason she started doing 2*2, 3*3, 4*4... that day she was busy with something else, so I did not mention those were square numbers.
Achievement test will certainly not ask for triangular numbers; and that is the type of math we do. In short, we do feed her brain and we do use learning opportunities, but we do not currently do direct teaching. However, her achievement scores were still very high.
In short, I guess that many HG+ kids will still continue to learn, they seem to be able to figure things out and acquire knowledge through osmosis.
I agree though, that a very bad school experience might slow them down. Also, like PP, extreme conditions could slow/stop a PG child's ability to progress.
