m1469, since this thread is kind of about you now-- and why shouldn't it be??? (hehe) yes, there are boundaries between teacher and student. When you think about your own students, the respect you have for who they are as people, their overall human potential as far as you can see it, and how much thought you put into those very things before you try to teach them anything at all... you would NOT talk about them to their friends. You would not tell them to drop those friends because the piano should be their only friend if they're really serious about music. I promise, this is not a criticism of your own relationship with your instrument, because people respect what you choose for yourself. But you would not choose those things for your students.
Where is the boundary? I think it's the line at which the relationship stops being healthy for the student. When students are made to feel uncomfortable by the conversation (unless it's regarding a failure to practice, then they get to feel uncomfortable). When students start to talk about themselves and go too far, and no one steps in to put them at ease. Touching their hand without permission, especially if they're shy. Reaching in front of them to demonstrate without excusing oneself/apologizing. Whatever! The teacher bears the responsibility for this, not the student. If the student is very assertive or the same age as the teacher, it helps the balance, but the burden is still on the teacher to behave respectfully and make the student comfortable enough to be free to concentrate on the work. We should act as teacher and, if they wish, respectful friend, and that is IT. No screaming, no guilt (except about not practicing, as mentioned), no imposing of ideals-- sharing of them, yes; manipulating life events to enforce them, uh, not so much.
Just to throw a glance at where this thread started out, might your screaming teacher be the same one as the guy announcing his orgasms to another poster?