Whenever I've taught anything where there were students who were way ahead of the others, I usually asked them to be my 'helpers'. Of course this may work better with elementary age kids than with adults. LOL!
The only crafty class I've taught with adults was English Smocking and all the students were newbies and eager to learn. Really enjoyed doing it. The other adult class I've been the teacher for was a Sunday School class and I told them I was the one who asked the hard questions and we were going to be discussing the material, not me lecturing. Have done some one on one time with people wanting to learn to sew. Find that I sometimes have a hard time stepping back and letting them do it themselves.