I just completed a couple of multi-level supplemental pieces for beginning orchestra. I really wish that more music came with more than one level of diffulty. When I first start my beginners, I have some in my class who have never touched an instrument, and some students who have already played for 1/2 a year. In order to keep everyone interested, I like to use multi-level pieces so that all can participate at the same time. I have found that this method also keeps the brand new beginners motivated to practice so that they can learn the 'hard' parts. My class moves at a quick pace thanks to the multi-level pieces that I have accumulated.
When teaching a piece with 3 parts for each instrument, I always start with the more basic part 3. That way, the advanced kids are helping the beginners and everyone can learn it very quickly. Once we have the basic part down, we add the more advanced parts.
One piece I just finished is called 'Caterpillar March.' Parts 2 and 3 are all open D or open A - so it is very easy for students who are just starting to read notes. Part 3 is pizzicato, and students get to slap the fingerboard for a little percussion effect during the rests. Part 2 is still just open strings, but with a more advanced rhythm and students are practicing smooth string crossings. Part 1 focuses on good hand shape and good intonation as the melody goes from G (all fingers down with good hand shape) down to open D.