This video uses some clever animation to describe the challenges of a cappella just intonation. I particularly like the way they use a honeycomb to visualize rows of just-tuned fifths, with each row being a syntonic comma higher or lower.
I frequently give a brief demonstration of tuning using a piano fifth and a sung third, followed by playing of the same note on the piano. This video gives the same shock in a powerful way. I showed the first few minutes to my students this week, and they responded well to the comedy and to the way the information was presented.
Tuning is a fascinating subject, and the finer points are something we don’t often have time for at the high school level–either because of time or because of complexity. I challenge you to take steps to expose your more advanced choirs to material like this, and then apply the knowledge it to the music they’re singing.