Somewhere between difficult and impossible.
Given normal distances and human reaction time, by the time the instructor figured out that something was going on, it was already too late to keep them together. Someone would need to be chased and someone would need to be ignored.
If a person bolts to the surface from 15 feet, do you really think you are going to catch him by chasing him to the surface? what level of genius does it take to take the others up with you when you surface to check on him?
As for the pool session that was skipped.....
Before the first dive of an OW pool session, or before the first part of a pool DSD, I tell them that we will work on basic skills in the shallow end of the pool, and we will not go to the deep end of the pool until we have done so. I explain the danger of bolting to the surface, and I explain that doing the first session in the shallow end, where they can stand up if they have a problem, is done in large part so that any problem with fear can be taken care of then. I explain the dangers of bolting to the surface carefully, and I tell them we are not going to the deep end of the pool until I am reasonably sure that won't happen with any of them.
I am absolutely stunned by the people in this thread that seem to think skipping the required shallow water pool session and going right to 15 feet in a lake with poor visibility was a reasonable instructor decision.