We all have a few of them. Stubborn kids. I was one of them!
The trouble is for those who practice, their parents are trained to help and disciplined about practice - the very last step is collaboration. Sometimes it doesn't happen because the student thinks they know more than their parent. And they probably do on most things, but not so often on practice strategies.
Anyway, I think I've solved this issue with the following phrase, "Mr. Julian said..." Try it and let me know. Email me at julian@oclef.com