Another factor at play with subject acceleration is the school's ability to differentiate. Reading is easier to differentiate, so a child who is advanced but less strong in reading might have needs met with differentiation. However if that students is very strong in math, which is harder to differentiate, the school may propose subject acceleration in math with differentiation in reading. This is probably ideal for the high achiever or the bright-and-balanced intelligent child who fits well with age peers socially.
My experience is that strong reading was overlooked. The school didn't seem to grasp that my son was as advanced in all subjects (science, social studies) as he was in reading because he was absorbing content so fast due to his strong reading. They treated his strong reading almost like a splinter skill that didn't require anything, rather than an indication that he was advanced in all subject areas. They offered single subject acceleration in math only, which prompted me to switch schools.
This is likely an effect of Common Core. Common Core changed the emphasis on reading from fiction to nonfiction (that's good), but in elementary years the Common Core model is built so that science and social studies are presented through reading and language arts. So, when children are being compared to above grade level in elementary years, the Common Core schools are really only looking at math and reading. I think this makes subject acceleration seem like a good solution in cases when a child would be better served by whole grade acceleration.
