Music Theory and History Online.
While the site design is very Web 1.0, the information contained therein is no less valid for being presented in a slightly less than currently fashionable manner. Perhaps the site's handiest feature is an alphabetical glossary of musical terms, each of which is linked to a detailed explanation of that term.
If you've had theory classes, much of the material will be familiar, but it's still nice to have it all conveniently indexed. And if you haven't formally studied theory, but suddenly need to know, say, what "acciaccatura" means, or the effective ranges of double reed instruments, they've got you covered.
For something completely different, in the embedded video window below we've got an hour-long master class from drummer Matt Wilson, who just happens to be in town this week doing an educational residency in the Webster Groves school district and playing a couple of shows with his Christmas Tree-O.
Although this particular class was recorded in 2013 for the Litchfield Jazz Festival in Connecticut, students in Webster no doubt will be equally appreciative of Wilson's extensive knowledge, down-to-earth approach, and sense of humor.