Autograph, I think you need to find a voice teacher in your local area and work with him or her on these questions. This is not something you can easily understand by reading in a book or on the internet. It has to be demonstrated in person to really "get it". Each person in your chorus has a physical potential and limitation to the range their voice is capable of singing, even after it is developed with good technique. Middle school students are not at an age when their voices are fully developed - their voices will change in puberty. Don't make them push their voices into a range that they cannot comfortably sing. Transpose the music to another key if necessary. If the kids are feeling shy, let each of them sing scales privately for you to find where their range is. If you want more detailed information about directing a children's chorus, you can email to gorders@mcn.net and tell her Alice Flynn referred you.

Alice