playing along in lessons

I think I probably do more playing aling with students during lessons than many teachers.  It depends on the age and level of the student.  With beginners, I tend to spend the first several weeks playing along.  I find it helps them to hear what they should be aiming for as far as sound, articulation and just learning to notice when they've played a wrong note or rhythm.  After a certain point, I start tapering off how much playing I do and try to get them to be more indepenent.  After they play whstever exercise we were working on, I'll ask them how they felt about what they played- get them to learn to be self-critical and be aware of what they're doing when they play.  After thay've assessed their playing, I'll concurr and/or add my remarks to point out what they did especially well, or bring to light an error they might have overlooked.  Some teachers get in the habit of having the student play something and merely point out the mistakes. I try to look at both positive and negative equally, so as to encourage students and  not make them feel like they don't do anything right.  

As students get more advanced, I tend to make duet playing part of the regular lesson structure.  Duets are a great way to reinforce ensemble playing skills such as listining to style, matching tone and pitch, and  learning to be able to play what you see on your line independent of what the other person is playing.  My life as a doubler taught me how important sight reading skills are.  I  admit that as a high school freshman, I was quite possibly  THE worst sightreader in the state.  At the time of my stroke.  I could show up at a rehearsal, read the music, get an idea of what I needed to touch up on in my own practice, and have the kinks worked out for the next rehearsal.  

Even with advancing students, I'll often play along during the early stage of just learning a piece or etude.  But once a certain degree of familiarity is acheived, I cut the cord and let them start playing on their own to see that they know what they're doing and aren't trying to crutch along.