I agree with what BillD is saying (btw. nice name Bill). We have been to a Celtic College for quite a few years now, there are different courses at Beginner, Intermediate and Advanced, even Master Classes, for ALL sorts of instruments and there are different singing classes in Harmony, Irish/Scottish/et, song. The song classes are ONE level, ANYBODY OF ANY Ability or experience.
In the evening there are organized beginners, intermediate and advanced sessions plus others where a few friends or people of similar abilities get together, this generally includes a Teachers session, where it is ALL semi/proffessional performers in the group.
Then there is the "Song Circle" where anything and anyone attends, some High tolerance people are there all the time, some with Blue Books, some with their own Folders of material and some who appear for a while, take their turn and then leave.
There are all the usual arguemants about books after, with the "Intermediate" and "Advanced" singers about stupid books and crutches etc.
The answer is obvious to all our problems, which is to recognize (in groups) the differing abilities of "Singing" not just "Playing"
JohnB