What happens to the kids that finish calculus in 8th grade and then have to sit in a class about algebra for beginners? How does it work during school?
The best online resource for math resources and courses is Art of Problem Solving (AoPS). Other on-line resources and courses include EPGY (Stanford), Thinkwell, CTY, NUMATS, and C-MITES. Many universities have free on-line courses in Math with outstanding Professors including MIT, Stanford, Berkeley, and Yale. Many students head to public magnet schools like Blair Magnet and TJ. Many students also dual enroll in community colleges and universities (for credit or non credit) while in high school to access more challenging work. There is no determined pathway, there are several paths that these students take. The good news is no bureaucracy is trying to eliminate the possible pathways for the spurious sake of social justice. There are some enlightened principals and teachers in our schools who recognise unique talent and make it possible for some of these students to use school class time to take these courses.