Have I had students whose parents (or selves, in the case of a few adults) enrolled them so they could learn "discipline"? Certainly. The culture of the class is such that, as long as only 1 or 2 truly undisciplined children come at a time... interestingly enough, most the students I've had enrolled who are "discipline" problems elsewhere are not problems in my class. I've had multiple students with psychiatric disorders that landed them in special education programs for emotionally disturbed students, even a couple who were hospitalized - and never had a problem with any of them; in fact, without exception, their parents reported improvement that extended to home and school while they were in class. And I never knew any of them had these problems from their behavior in class; only after they'd been in class a few weeks or months did their parents tell me about their problems. It's why I'll let any student try - but rarely, there's one I won't let stay.
The ones who are are generally the worst are the truly spoiled ones - like this one:
I had a student rather like the one Tez described - not quite as bad, as he made it to red belt - but around blue belt he decided he didn't want to be there any more... and his parents told him he was too close to black belt and wouldn't let him quit. This went over so well that one day when they dropped him off at the door of the Y, he waited until they left and ran across the street to the shopping plaza, hung out for 90 minutes, and came back - slightly late - which is how they caught him. He was 11 or 12 at the time. He's also one who's parents would drop off his younger brother "to see if he's ready to start yet" every time they needed a babysitter and couldn't get one (that lasted til the 3rd time, when I refused to let him into class to mess around, and left him in the front room, unattended - safe enough, as we're in the basement, but still not what they wanted for their 7 year-old). From about blue belt onward, even though he was reasonably good for his age, if it wasn't a game or a contest, I couldn't get him to perform. That lasted until he tested for 1st gup red belt, and failed - not because he couldn't perform appropriately, but because he couldn't be bothered to demonstrate that he could perform properly - one of the very few students I've ever had who failed a test. His parents came in, very upset that he'd been failed, and asked why - so I went over every area of the test, explained his scores, and (as I had been instructed to do) provided them with the testing instructor's phone and email if they had any questions. After dad and son discussed their next step for about 10 minutes, I reminded them that dues were due that evening (another problem I'd had with that family was that they only paid dues when I started leaving the student in that same front room when they didn't pay - which was about every other session) - I've never seen him since. Another student of mine whose kids went to the same school told me the boy went steadily downhill after that, which is too bad - he was a good kid, but horribly spoiled, in terms of being allowed to have his own way all the time - but as is too often true, it wasn't just the child who needed discipline, even moreso, it was the parents.