While most subjects must be taught face to face, music performance is successfully taught online. Is anyone else doing  this? I tutor a score of folks each week this way now
 from my website, some of them local (even through 
Wyzant), but many more of them are  from all over the world from places I could never physically visit. 
 Instead of connecting to a student in a physical space, we connect in a virtual space. My student and I are both at home  on our own computers, with an audio headset, a common chat channel and sometimes a mini-camera active. We can tutor over  Google-Talk, Second Life, There, MS Chat, or Yahoo Messenger, but my favorite music-teaching platform is Skype. Skype  service is free for all, the sound and video stream is fantastic and I can inhibit new callers so the student and I are  not distracted by incoming calls during our session. 
 I always spend the first few moments of the tutoring session making a good connection so we can hear and see each other  clearly. If static, break-up or background noises interfere at all, we spend a few moments taking care of those, checking  connections and setting levels.   After that, the online tutoring session is very like a live face-to-face tutoring session. 
 There are some differences, of course. We can see and hear each other clearly, but I cannot point to a symbol on a physical  paper page to direct attention to a detail. I cannot touch the student to correct a posture or hand position. However, I can  instantly send a link to a website video, picture, or article that we can look at together and talk about right then. I can  chat notes about an exercise while the student is singing or playing without distracting her and send it immediately on  completing without distracting her from focusing on her performance technique. Using a recording program, I can make and  upload short clips of our session so the student can review them after our session.  In trade off, I think the online session  offers the student more resources than the face-to-face. 
 I do not think this practice is widespread yet, but it’s effective and economical for teaching vocal and instrumental  practice and technique.