[The students] really enjoyed the work you did with them, and I heard them recently speak of it in a positive light (although it was weeks ago).

It was especially impressive that you were able to work so efficiently in an impromptu situation–without knowing what the students would sing, or in which style.  I believe it was only suggested that morning!  Despite this, you provided meaningful feedback for them to process.  The time limit was another challenge you overcame by your ability to provide quick and positive feedback.