My kids were not particularly advanced (none beyond linear algebra in high school), though every year there are students who are quite advanced in math in our school district.
The answer is going to depend on what your school and area offers in terms of math. See what courses are offered at your high school. Our district has a bus that goes from the middle school to the high school each morning (or they did, when mine were in school) for kids who were advanced in math.
Some years in our district there is a student who runs through all of the high school math courses while still in middle school (so through multivariable calculus). My eldest and middle children each had a student like that in their class. Fortunately, there are a number of four year colleges within a couple of miles of the high school, so they took math classes at a local college.
Your child might get involved with math competitions. The student in my eldest child's class that ran out of HS math classes after 8th grade made USAMO every year from 8th grade on - those types of students are rare, of course, and most students are not that advanced. There are usually other math competition opportunities in high school, such as a math team.
I'm sure there are a number of online options that others could suggest - Stanford Online High School comes to mind, and a number of colleges offer online courses.
If your child is advanced in just math, I wouldn't be too concerned about the social aspect. They might be with older kids in the math class, but with students their age in other classes and extracurriculars. Good luck - there should be options, either in your school district, at a nearby college or online.
