Coursera is not ONE course, but an entire site of univertiy level classes from places like Stanford, Princeton, Penn. Many are not for the faint-of-heart, and for most students will be above HS level. However some may be accessible, so have a go.
From "A History of the World since 1300" through "Gamification" to "Vaccines", there might be something for you.
Teachers: If you approve a particular Coursera course, please add it individually as an "Approved" Course.
Video introductions to most of the courses can also be found here: http://www.youtube.com/user/CourseraVideos| All courses