I’ve had problems with several of my students sticking out, for different reasons:
1. Singer isn’t on pitch. Usually this is the case when a student hasn’t found his head voice so I work on that.
2. Singing louder than everyone else. If you have created a safe environment, you can probably tell someone gently (privately or even in the group setting, if they are not easily hurt) that you don’t need them as loud right there or tell the group to make sure they can hear those around them.
3. Strident voices. For some reason, two of my most talented boys stick out when they are on soprano even though it is fully within their vocal range. When I moved them to a lower harmony line, they blended in fine.
On Laudate Mennonite Ensemble this past fall, Ken worked on blend during warm up exercises. He’d have us all hum the melody line and walk around randomly while listening to others or stand in different voice groupings. I’m working on students mastering their part sufficiently that they can be independent, standing by people singing different parts. Moving singers around produces a beautiful, fuller sound.
I think singing with a straight tone is important for me personally to blend, but this probably isn’t an issue with school-age children.