Many thanks for comments so far - I'm reminded, more pithily, of useful advice I was given in my own teaching days (though more often from colleagues than from college lecturers).
Mo -- "private consultations" would obviously be a great idea, but I'm not sure if time will permit. And we've yet to discover how self-aware the participants are.
Crow Sister -- Many of our participants have been listeners in sessions for a long time, and I know that several have entered singing competitions at traditional music festivals for the feedback, rather than for possible award.
I suppose Gordeanna and I just wanted to try to replicate the supportive atmosphere of our own introduction to traditional music in the late 50's/early 60's at Rutherglen Academy Ballads Club (just outside Glasgow, Scotland) under the leadership of the late Norman Buchan. In that group, no-one was ever told they couldn't sing, and every effort -- as I realised later -- was made to find a song that would suit a person's needs and abilities, working on the theory that success in one area would build confidence to tackle other types of material.
But we were weans then, and more used to the idea of advice/criticism/being told what to do. And now we're dealing with adults, which is a different matter.
So, please keep any suggestions coming, especially the ones that you know have worked because you were on the receiving end.