Beginners
If your class have only begun learning the tin whistle, you might want to stick to something simple and repetitive. When I am teaching tin whistle to a class for the first time, I focus on the BAGFED notes until they know these very well. A tune like Jingle Bells is ideal for a beginner class as it only features the BAGFED notes.
Intermediate
If your class are familiar and comfortable with all of the notes (including C natural), they could play Oh Christmas Tree or Rudolph the Red-Nosed Reindeer. These tunes are a little more challenging than Jingle Bells, but are still quite repetitive and easy to learn. Away in a Manger is another short, repetitive piece that is quite easy once your class can play every note.
Advanced/Popular Christmas Songs
If your class have been playing the tin whistle for a few years, they may have played a lot of the more common Christmas tin whistle tunes. If this is the case, try teaching them It's Beginning to Look a Lot Like Christmas, Chestnuts Roasting on an Open Fire or Oh Come All Ye Faithful. This version of Silent Night features a harmony that can be taught to more able students for a class performance. You could also teach them Do You Want to Build a Snowman or Let it Go (if you can bear it!) for something a little different!
These are just a few suggestions of Christmas tunes your class could learn on the tin whistle in the run up to Christmas. If you are interested in more, check out Nollaigshona.ie - you can find a huge bank of traditional Christmas Hymns and carols there too.
If you are interested in previous posts from my tin whistle series, you can find them by clicking on the following links:
Teaching Tin Whistle Tunes for a School Mass
Teaching Tin Whistle to Beginners
Merry Christmas!