Balancing choral sounds through repetition will not only help your group to obtain the desired audio result, it will also foster an environment of working together. What other class in school requires listening to everyone else--at the same time?! Music education is undoubtedly the definition of teamwork at its best because it takes all the participants to achieve the right sound. And this right sound is achieved by balancing.
To begin, it is always important to explain to your choirs the value of balancing sound and that it does not mean that all parts should be equally as loud. A great saying to explain this concept is, "while all parts are equally important, they are not all the same." Sometimes certain notes need to stand out more and some need to hang out in the background. Learning which and when is key.
Here are some examples:
Even the most musically challenged person will appreciate a balanced sound. There is something so natural about having the right mix of parts and the degree to which they are audibly pronounced.
It is amazing how young choir members understand these concepts straight away. The challenge of course is to have them follow through and do something about it. Sometimes recording the group is the best way for them to understand how they are projecting their voices and where the balancing needs attention.
These concepts may seem a bit advanced, but at some level, students do understand them. With consistent practice, they will develop into much more sensitive listeners and performers.