Allyphoe, I think you should discuss this with your spouse. In your situation, I think I would opt not to jump into boarding. It sounds as if your daughter will be in a better position at the end of high school if she stays with you. Being the weakest kid in the group might be best for her from a purely developmental standpoint, but it would be unlikely to place her in an appropriate college. You would run the risk of her gifts being underweighted by colleges, given her standing in class. The boarding schools generally don’t provide class rank to colleges, but the colleges do apparently compare students to their classmates.
Are there any options in your area for cross-registration at local colleges? That would provide her with academic challenge, but allow her and you to set up a system in which she can seek out as much challenge as she can handle, given her other commitments. Boarding schools are challenging, but unlike living at home the day is very structured. There are not large blocks of time students could use to catch up or to get ahead of classmates by working harder. Spending all free time on academics is a losing strategy, as colleges expect students to have extracurriculars and social involvement in their boarding community.