Take a rehearsal and walk them through the process (see below). Do each part and then discuss after so they understand. Show them the different exercises in a warm up they can do specific for their instrument and explain the difference between a full ensemble and an individual warm up. Show them how to break down a 12 bar chunk of an etude starting in bar 12 and working backwards or in 4 bar chunks etc. Show them how to cool down. Make practice less about you and more about them, (It's time consuming to go over basic fingerings in full ensemble but that's your problem and not theirs). But to them, that's what rehearsal is. Once they understand that rehearsal is for the ensemble and practice is for the individual some of that will improve but it has to be about THEM.
And learning Region etudes for auditions isn't fun and doesn't motivate anyone to practice. I like to pull out the books the etudes are pulled from and practice. But the average 14 year old doesn't because they associate it with 8 hours of an audition. Assign different etudes throughout the year and see the mental change.
Talk and have them do the following:
1. Plan a practice session (takes less than a minute to plan it out once they know how)
2. Warm up (5 minutes)
3. Fundamentals (scales, tongue/finger patters, arpeggios, etc.
4. The meat of the session (Literature, etudes, region work, new fundamental exercises that move up to fundamentals later)
5. Fun (why they play the instrument to begin with)
6. Cool down