I'm reading the book "Love for Learning Motivation and the Gifted Child" by Whitney and Hirsch.
Quote
Clustering for bright children may be of 2 basic types: (1) a "talent cluster" for those who far exceed the academic norms for their grade in a particular discipline, such as math or science, and (2) a "gifted cluster," for those who are strong in abstract thinking and problem solving. In a talent cluster students are identified by achievement tests, and the accommodations made for them may be restricted only to acceleration in their specific subjects. This type of cluster can be a daily class in the particular discipline.

the gifted cluster would include students who have demonstrated intellectual giftedness on ability tests and other measures. They might be achievers with high grades, but the group could also include under-achieving gifted students who teachers, parents, and counselors feel would benefit from exposure to a deeper, broader curriculum. This group would carry out more independent and problem-based study. The differentiation of their curriculum would probably included some acceleration, but it would primarily allow for more in depth examination of the subjects that the entire class is studying.

These two types of clusters are quite different from one another, and participants must be chosen carefully. Some gifted students might not be able to maintain the speedy pace of the single-subject accelerated students who generally whiz through the curriculum. The talented students may balk at the slower pace required for studying a problem or issue from multiple perspectives.

I found the above passage interesting. I also read this in another book and it's what I often think about when someone suggests to me (like DH) that I should talk to the school about having DSs skip a grade. Especially for DS9, I think it's more the 2nd type above - the "gifted cluster" but he needs both - deeper and acceleration.