What do students expect to do?

Well, this happens every now and then. Students go to violin lesson, or piano, or any other instrumental lessons, without preparing for it. That is, they have not been practicing at all for the past week, or longer. Yet they go to lesson anyway. Most of the time, these are children who go for lessons because parents say so. Meaning, they have no choice.
Well, actually they do have choices. Perhaps the most known choice would be, to practice before going for lesson.
Anyway, what can the teachers do, if the students were not prepared? I used to just think, to let them practice during lesson. If there is no improvement (due to lack of practice), there would not be progress, and it would prove difficult to learn any new skill, or material, or just another new piece.
Is there other alternatives? I should have given this a little more thought. Maybe I will just spend the lesson time to practice, not the expected prepared piece, but exercises for basic skills. It is no substitute for regular everyday practice, but at least the lesson would not be a total waste. I wonder who else is reading this blog, and if any would care to comment.