I've had students cry in the middle of their lesson in the past year. Usually I regard that as a complete failure on my part. It usually comes from applying too much pressure on students who are not aiming at being professional musicians. I've come to expect a certain level from my college students, the ones that want to make a living as musicians. Sometimes, when one of my other, younger, students starts slacking off, I have a hard time changing gears and treat them as I would treat a career musician (especially when their lessons are right next to each other). I yell and I mock them until they crack and then, I feel like dirt.
If you have a student that is generally responsible and respectful, but isn't looking for a professional career, what is the use in getting angry, and making him feel bad? It took me a while to come to that conclusion, but once made, it puts a lot of things into perspective. As a teacher, I can only have a couple of students break down and cry in the middle of a lesson before I have to stop and try and figure out what I'm doing wrong.
If a teacher is not willing to work at anything less than professional level, then he should not take other kinds of students and expect them to work at that level all the time.
I had to go back and think about each student's situation; why is he studying with me? is someone else making him do it? what does he expect from the class? how much time does he reasonably dedicate to the piano?
You can't go into each lesson and act like one of those old-school Russian teachers, throwing tantrums and smashing furniture into the walls; expecting everything to be absolutely perfect and every single student practicing eight hours a day. There are moments when that kind of attitude might be justified--- maybe if one of the more professional students is getting lazy with absolutely no good reason--- but for the most part, it is a lazy way to teach, because there is no actual teaching going on. Just bullying.
Even so, bullying works to some extent. If you doubt that, just look at Cuba or the level of piano playing that came out of communist Russia. There were some amazing musicians teaching there but, as a few of my teachers used to say--- a couple of them a product of the Neuhaus era Moscow Conservatory--- a big part of the success of the "Russian school" of piano from back then was more the result of coercion and an extremely oppressive environment than of any specific kind of technique.
In short, if you take students that are not going to grow up to be professional musicians, make sure you adapt. I'm not saying that you should dumb things down, but when they don't study as much as a career musician would, it at least shouldn't surprise you.