I would introduce TRANSPOSITION by placing three simple melodies on the board, asking the class if they could name the tunes.  [Twinkle, Twinkle - Jingle Bells - Joy to the World].  Then I would explain the concept of transposition, especially its importance for a singer.  Further explanation included the ability to transpose by interval (time consuming and the long way) or by key [faster and more efficient].

Assignment-SHENANDOAH:  The printed score and the transposition should look identical, line for line - adding lyrics and all markings.  The practice of transposition is critical to the life of every musician.  

Once completed, I would play their transposed SHENANDOAH  for the class.