Abandoned America: Cars + Homework in the Art Room?

About a year ago I was connected with an art teacher who retired a couple years back.  From all of her many years of teaching all grade levels she kept one box of resources.  That coveted box was then transferred onto me.  Every once in a while when I'm looking for a new spark to a lesson and am not feeling the inspiration from the web, I'll pull something out of the box.  The inspiration for this lesson came from a newspaper clipping from that box.  On the clipping, was a photograph of an abandoned car done by Steve Gottlieb.  So I did a little searching and came across this book, I had to order it and once I received it I was just flooded with ideas!  I had a few 6th grade boys previously ask about doing a car project, but didn't really think much of it until I came across the work of Steve Gottlieb.  

I decided to stick with the idea of Abandoned America: Cars Edition.  I created packets of about 20 different "classic" style cars that students could choose from, we viewed these images to give students inspiration as to where their car "might be". 

I was so impressed with the drawings (the details!!!) 6th graders were creating and the scenes they were coming up with.  I didn't limit what the students could do in regards to their background, it could be fantasy or realistic I really left that decision up to the students.  

I was so thrilled with results of the drawings, but I was a little concerned about how to add color to these beautiful drawings.  Because of all the details I went with the colored pencil route, and let me tell you I loathe colored pencils.  I hate that they have to be sharpened every ten seconds, sure they can create a lovely finished product, but to get there takes a lot of patience and work.  Nonetheless I still did it.  

Now here comes the problem.  Remember I only see each group of students once on an eight day rotation, we started these drawings before Christmas and just finished them last week.  We'd still be working on them, but it was given as homework.  I hate to do it, because the quality of the work often is not turned in at the maximum level, but how many other classes let's a student work on one assignment for 50 minutes 5 classes in a row? Especially sometimes with 5th and 6th graders it needs to be done or we would never accomplish anything.

Here's the kicker, our next project was a clay project, so students who did not turn in their work or had not completed it do not move on until the drawing was done.  I had about 5 of 175 total students who instead researched about our clay project instead of actually creating one. 

How do you feel about homework in the art room? When the needed supply enables a project to be finished in the classroom during "free time" or taken home, is that okay?