Why do boys have to stop singing in the school choir?

Boys' voices change when they reach puberty to become deeper like men's voices. The expression used is that their voices "break". To begin with the voice has trouble changing since it's sort of half way between a boy's voice and a man's voice. Boys whose voices are breaking find it hard to sing properly because their voices go up and down like a yoyo. But that phase doesn't last very long.