I agree with the excellent insights of the other posters. I can also contribute a "success" data point, with the caveat that our local public school may be unique.
My twins are in their first year of middle school - 6th grade. They are not grade accelerated (12th birthday this month). However, they are 3 years subject accelerated in math (Honors Alg. 1). They also have a 6th grade peer (similar age) 5 years accelerated in math (currently taking Algebra 2/Trig.). None of these kids are slouches in other academic areas (e.g., dd's recent reading MAP score was at the 99th percentile for 8th graders), but the school seems to do okay with accommodating through cluster grouping and differentiation.
(By the way, they offer Alg. 2/Trig. in a regular class at this middle school, but for levels of math beyond that, kids would have to take the bus to the high school 2 miles away.)
