When you think about it, just about any dive an instructor does with students is a solo dive.
So you are an instructor and let’s say you have 6 students on OW dive 1. Perfectly fine, right?
Which one of those 6 students will be able to assist that instructor in any way if there is a problem.
It’s a solo dive.
As for the quarry: they appear risk averse so i’m sure you will never get them to admit to this policy. Sometimes a good way to reduce liability is not monitor what people do TOO closely when they hit the water.