I don't know if it will help, but one of the things I've found is that learning music by ear (alongside the book-learning) can be a great leveller in this situation, particularly if it's the difference in the competency of music-reading that is dividing the group We can direct a different level of challenge to the pupils, depending upon whether they feel happiest learning a melody line or, more simply, an accompanying ostinato - either way, it's good fun to make music together!
PS my pupils love a Christmas tune too!
PS my pupils love a Christmas tune too!