Did you ever find an arrangement? We did this one by ear.
I agree that 6th graders can't produce the ideal sound and quality this work of art demands, however, based on my view of Music Education I really believe that it's good for young students to learn and perform classics to the best of their ability. I've heard (and played for them) several good examples of young choirs singing this song wonderfully and they had that in their minds as they tried to learn how to use their voices for the first time.
No. I used to teach there.