I'll add some experiences from my DD15, who will also be in Calculus next year in 10th grade.
We had opportunities to accelerate her even further, but after I read
The Calculus Trap, by Richard Rusczyk, and received some advice from
an old college fraternity brother of mine, now a math professor at Harvey Mudd, we decided to focus on enrichment rather than further acceleration.
There are a couple of kids who are one year ahead of her, and one who is two years ahead. Yet on SAT Math, Mathcounts, AMC, etc., she outperforms them. Mostly because we have focused on learning Number Theory, Combinatorics, Discrete Math, etc. to broaden her math knowledge, rather then driving fast through calculus. Yes, they can integrate and differentiate, but they haven't a clue how apply Cauchy-Schwarz, leverage Ptolomy's Theorom on cyclic quadrilaterals, understand the statistics and probabilities that drive every-day decisions, etc.
I recently gave her a quick run through matrices, which she hasn't studied yet, for the Math 2 Subject Test. She was intrigued enough by it to start on online Linear Algebra class to learn more. One of her math teacher's comments was that she displayed a need to fully understand the details behind new topics, not just learn the formulas. I believe that her driving need to understand and learn new subjects has been driven by exposure to the more elegant areas of math.
IME, exploring areas outside the traditional sequence and developing an interest in the broad topic of math, rather than racing through courses, has been beneficial.
So when a school is opposed to rapid acceleration, look to enrichment opportunities outside of school. Or even if they don't.
