I’m definitely not an expert, but I’ll give my opinion.
I am actually in a similar situation as a 10th grader whose main interest is math. I applied for 11th grade without think about doing a repeat year, which now seems like a great option. Boarding school would provide me with an experience I won’t get any other time in life, even in college. I wouldn’t consider it “wasting a year.”
I have a lot of experience with contests. One of the reasons I want to go is to be surrounded with people who can share this interest. Your math ability is only as important as what you want to do with it and displaying interest.
The statistics are very intimidating for 11th grade. At this point it’s best to not think too much and wait.