I attended Northwestern University's summer programs for gifted middle schoolers many (many) years ago. I just learned the university now offers similar programs online during the school year. If you're looking for another distance learning option, see the LearningLinks program webpage. You can also read a story in the Daily Northwestern about the program.
These programs are ideal for kids in small high schools that can't offer many advanced courses. You simply go to the library and log into AP Statistics instead of showing up in class for Math 9. At about $550 a course, it's not cheap, but if you've only got 2-3 kids who need such work in your school, it's cheaper than hiring a teacher.
Of course, my favorite part of Northwestern's "nerd camp" (as such programs are affectionately known among attendees) was hanging out with other kids like me. I liked the academics, but I really liked the social aspect. Being online isn't quite the same. Still it's better than nothing. A lot better.