One of the hardest parts of teaching for me is deciding how to criticize a student's playing, practice or performance. The critique needs to happen. The student grows from learning how to make their piano playing better. Here are 3 things I've been trying to take into account lately.
Privately - no one likes to be called on something in an obviously public way. In my opinion, it is not acceptable to call the parent into the room to berate the student's ability, practice habits or attitude. I would like to respect the student as much as possible, mention it until it is resolved and will choose not to bring it up in group lessons or in writing if I can absolutely help it. I inwardly groan when I hear adults speak painfully of their embarrassment in front of peers toward a music mistake. How demeaning to be told you're tone deaf in front of the rest of the choir! That child carries that baggage with them perhaps forever.
Self-realizing - I have become a big fan of the student hearing and realizing the mistake on their own. There are a few tricks to doing this successfully. I have played the piece with them. I have 2 pianos here; one for each of us. When they hear that we are not playing it the same, they stop and look at me. Like a puppy cocking his head to the side, they sometimes ask why I played it wrong! Or, I have asked them to become the teacher. Then we record the piece and use the music to really listen for what they would tell this "student", who is in fact, themselves. And I just love the new iTunes approach. We go online and buy a ninety-nine cent artist's concept of their piece. I challenge them to listen carefully. I ask them perhaps to purchase a similar artist, talking about some of the great pianists listed. They then have an ear for the potential music of the piece. Many students do not listen to classical repertoire at home. They don't know what to listen for.
Gently, with humor and grace - One of the best parts of learning happens when they realize mistakes happen. I have been known to murder a phrase of their piece in their lesson. I trip, stumble over the notes, count incorrectly, and we laugh. I have shown them my music, marked beyond recognition in some "hard parts" and they realize over time that we are in a state of becoming. We are all learning.
I won't give up. I can't. They are SO worth it. I will bring it up one more time, in another way, make a crash-and-burn sound, oooooh noooo, "dun, dun, DUN!", or the famous question, hmmmm, "how exactly did you practice this?" We'll get there, we'll get there.