What is the outcome you are after?
Once you decide on that, keep changing your behaviour until you get that outcome. The main problem with this approach can be summarised in three items:
1. People in general do not know what is the outcome they want.
2. Even when they know the outcome they want, they lack a large enough repertory of behaviours so that they can change it.
3. Many times people know what they want, they hit upon a behaviour that gets them the desired outcome, but they don’t notice it.
So it’s up to you to figure out what you want, and it is up to you to notice if and when you get it. But I can suggest a number of behaviours you may be willing to try.
She arrives at the lesson. She sits at the piano. She has not practised.
1. Finish the lesson there an then. Tell her: I am waiting for you. Come back nest week with the material ready so that we can proceed. I would love to proceed, but there is really nothing that I can do. Don’t be angry or upset. Be matter of fact. You are dealing with a law of nature, not a moral or ethical issue. Next week, if again she has not practised, send her back again with a letter to the parents explaining the situation.
2. Leave the room (go read the newspaper, watch TV, post to the forum, you name it) and tell her to practise the piece. At the end of her allotted time come back and see how she is. Repeat next week if necessary. Again, no anger or reprimand is necessary. Just a matter of fact attitude that this is how the Universe works and there is nothing anyone can do about it.
3. Instead of a piano lesson do a therapy session. Enquire what is the problem. Is she having trouble at school, family problems. Get to the bottom of the question. Be compassionate and understanding.
4. Sit down with her and practice with her. Go through all the steps. Playing everything two octaves above her so that she has to follow you. You will be teaching by imitation. The attitude here is of infinite patience. Pretend she has severe learning disabilities. Feel sorry for her.
5. Repeat your last lesson as if she had never had it before. Again be patient and enthusiastic. Pretend she is a new student who has never heard any of this. Repeat the next week. Keep repeating until either she learns or she can’t take it anymore. Explain that there is nothing you can do about it, since for the lesson to change and the matter to advance she must come up with the goods.
6. Encourage her not to practice. Say to her: I myself don’t really like to teach the piano. Should we play cards instead (and do just that). Please don’t practise or I may have to teach you something new, which I dread. Please come back next week not having practised again.
7. Sit at the piano and ask her to sit on your chair. Then spend the whole lesson practising the piece yourself. At the end of the lesson tell her, there, I practised it for you. At this point offer her a glass of water. If she accepts, bring her a glass of water, and drink it yourself. Then ask: What are you still thirsty? But I just drank all that water, how can you still be thirsty?
8. Tell her you are bored with having to teach her the same thing week after week. Tell her you don’t mind her being bored when practising, but by escaping her boredom she is making you bored. And that is definitely no no no.
9. Tell her that the main point of learning the piano is to have fun, If she is not having fun then maybe you should assign her a different piece.
10. Scream at her. Tell her angrily that you are not going to have any of this nonsense anymore. Act possessed.
11. Be extremely nice. Tell her it does not matter, and that we all had bad weeks, then proceed with the lesson as if nothing had happened.
12. Tell her practice is exciting and fun. Then prove it. How? I have no idea. I never tell my students that practice is fun. I always tell them it is hard work. But if you believe it is fun, and you can prove, then go ahead and try this option.
There are many more options, but you get the idea.
Now the really important question is this. Did any of these options shock you? Did any of these options go completely against your philosophy of teaching or your moral and ethical principles? If your answer is yes, you are severely limited in what you will be able to accomplish.
The fact is, I have no idea which of these behaviours will get you the outcome you want. But it might just be the behaviour you could not bring yourself to do. Does that make sense?
Finally. How would you know which behaviour would be appropriate? Simple answer: Good judgement. And how do you get good judgement? Experience. And how do you get experience? Bad judgement!

Best wishes,
Bernhard.