I don’t follow? 9th grade English is not varsity tennis. 8th grade English is not middle school PE. For one thing, there isn’t a limit on the number of spots on the 9th grade college prep English team. There is room for everyone to learn 9th grade English, especially if they have already successfully completed 8th grade English.
As for the Stanford comparison, you really lost me there.
If your kids are the next Stephen Hawking, Albert Einstein, or Ruth Bader Ginsburg, I’m sure they’ll be able to get more out of the course than you imagine. And think of the the gift you are giving to the other kids by allowing them to be graced with your child’s presence. 
I’ve always known that CC parents think very highly of there own kids’ abilities, and I am no different in that regard. But it is disappointing to see how little they think of the abilities of everyone else. We are talking 9th grade English here. Your kids will survive if they don’t get their own special class insulated from the undesirables.