By profession, I am a teacher. I teach choir to 6th, 7th, and 8th graders. Given all that happens in middle school, singing can be a challenge. You have changing voices (or, worse for some kids: unchanging voices), the pressure to look cool, shifting social landscapes, evolving relationships with members of the opposite sex, and on and on. All of these things can make singing extremely difficult.
In light of that, I have noticed an interesting phenomenon. I remember one year I had a class that just sounded awful. We worked and worked and worked and worked and by the time the performance came, we did a pretty decent job. It wasn't world class, but it wasn't bad. In fact, it was pretty good considering our progress.