This hypothetical group sounds eerily familiar - do they speak with a British accent? Ah, you beat me to the punch, LF
At a ski area where I used to work, we had an instructor who got people moving down the hill quickly by teaching them Jersey turns. That's where the skier violently rotates his upper body leading with the hand, almost like throwing a punch. The legs can't help but follow. Oh boy, try to unteach that later. The only solution is AMPUTATION - of the arms at the shoulder.
I try to teach beginners only things that they will never have to unlearn. Even in a group of that size, I think that is the best way to go, since this is a multi-day group. The size dictates that straight to parallel is likely to be unsuccessful for at least some members in the group. You know, as in demolition derby. So I would likely be teaching wedges at the beginning to give the laggers the most opportunity for success and not hold back the rest of the group too much.
Choose your terrain well, and use what you have to use wisely. The guilty party was teaching his wedges on terrain that was too steep, and they were going straight down the fall line. It wasn't particularly safe.
I think it is just as important here to teach good fundamentals as it would be with a single person or a small class. I have always admired the wedge as a teaching tool because of its inherent stability, simplicity, and the luxury of time it can allow the student to master the movements. You can add just about anything to it. I start by teaching them to balance over the new outside ski and that the new turn should begin above the fall line. That won't change as they evolve, and they establish their turn above the fall line with more time to complete the movements that are part of a turn. The turn has shape and completion.
As you progress in teaching the wedge, if you have given them good fundamentals, then a natural movement into parallel will be a result, sometimes before it is an intention.
The other thing is that as you move to steeper and more challenging terrain with these kids, the bad movements they have learned will come back to haunt you.