Parental awareness of just how much time students need to practice will help things immensely.
Eventually, when students are mature enough, they will stop watching the clock and do the work demanded by their assigned material to make sure it is prepared. Remember this is not wishful thinking on my part. There are lots of hours in the day; most of you just spend them watching tv, playing video games, on facebook, etc. If you budgeted your time wisely these practice times would be easily achievable.
Below are recommend time lengths for students at different levels (again these are recommendations, occasionally for some students the material for a lesson may take more work). Please do not feel like you have to do all this practice in one session. Often two or three shorter sessions are more effective (e.g. two 45 min, or three 30 min sessions instead of going for 1 1/2 hours at a one time).
6th-7th Grade 15-30 minutes 5 days/week
8th-9th Grade 30-60 minutes 5 days/week
10th-12th Grade 1-2 hours 5 days/week
10th-12th Graders with an interest in receiving college scholarships or who are planning on majoring/minoring in music should plan on practicing more time, something like 2 or more hours 6 days/week. Music is a discipline as much as an art. I cannot guarantee that you will succeed with hard work, but I can guarantee that without it you have no chance.
Something else that I've found very helpful is to realize that you cannot 'cram' for a lesson like you could a test. Do not try and practice the day before your lesson for 5 hours. That is not good for you physically, mentally or musically. You will progress much faster with DAILY practice. Even if that means only 10 minuets one day, an hour the next and 30 minuets the following day. Another way to think about this is that 30 min every day is more beneficial than 2 hours every other day. You can get more done with half the work. Doesn't that sound like a good idea?