I'm glad someone mentioned EarSketch. As it's more focused on teaching programming and computing principles by making music, as opposed to a general purpose computer music language/environment like Chuck, Pure Data, and Supercollider. I was involved with the project during Grad school when it was completely attached to a DAW (Reaper), but it can now run in a browser. I'm usually not that excited about audio applications in a browser, but I think this is a great use case.