We had similar issues with DS elementary school so we supplemented with AOPS. Eventually, he studied independently during math class and attended monthly math circle meetings. The math circle was involved in competition math which he started doing.
He completed Calc BC last year in 9th and this year is doing AP Stats plus involved in a Linear Algebra research project at a local college. Each year we wonder how we will keep him challenged in math but something works out.
It was not an ideal path but it has worked for him and better than the alternative the school offered. Even if the school had more to offer, it would not have been nearly as challenging for him as what he has done.
