Since you are looking for materials for teenagers (who likely prefer the an online experience), you might consider a learning forum such as NauticEd. http://www.nauticed.org
NauticEd offers a variety of learning skills--from beginner to experienced--in a wide array of sailing subjects; a look at their site gives a glimpse of their online courses. These courses are easy to understand and provide an excellent opportunity for teenagers, or adults, to learn about/how to sail at their own pace.
While online education is a good start, nothing beats hands-on experience to hone skills.