I can tell when a student comes in unprepared, either through their scales that they were to have memorized or through their solo that had not improved much (if at all) since the lesson prior. It happens to all lesson teachers and it is a matter that is difficult to address.
As a teacher, you could think of strategic ways of getting your students to practice, but what is the point in that? You know it won’t work for every student and you definitely don’t want to force them into practicing for fear the student may dislike learning to play the flute or whatever musical instrument they are studying.
I always remind myself that I can’t be there for my students every single day of the week to guide them through their practice sessions; that is something they must do on their own. It is called self-discipline and yes- it is difficult to learn at a young age. But practicing is a good habit to develop and in order to develop such a good habit, I tell my students this:
“It doesn’t matter how LONG you practice, just be sure you practice EFFICIENTLY”
Anyone could word this differently and I know I do on occasion, but it still means the same thing. Even as a professional flutist, I find days where I am just swamped with teaching, rehearsing and concerts that I do not have much time to practice, let alone take care of myself. So on a day where I have a heavy load, I make sure I find pockets of time to practice. For example, I could have twenty minutes in the morning before I leave to a rehearsal, so I do some warmups. Warmups may consist of long tones, harmonics, scales and technique exercises. Then I find time after dinner for about fifteen minutes. It may not be a lot of time, but it is enough to work on the challenging Köhler etude I have been struggling to get up to tempo cleanly.
As for my students, I make sure they have goals set for themselves as a growing musician to help them get the drive to practice regularly. Their goals may be to do better in solo and ensemble next year or get first chair in their school band. I also make lessons fun and enjoyable; typically, my students like to impress me with their knowledge and that keeps them practicing as they are happy when I am happy with their performance in our lessons together. They also know that I will keep reassigning material if they fail to meet the standards in our lessons together, and it is no fun being held back because of poor effort spent in the practice room. Sometimes parents also do not know the value in practicing on a regular basis and this is why I am sure to include a sample practice guide in my policy packet, which features a chart with age range (years of playing) and how much practice time is expected of each student.
Overall, I would say do not force practicing upon your students, but suggest a practice schedule to them. Show them how much work is needed to be at a certain level in their playing. And remember to place them in a good routine!