The instructor needs to a little more reading on canine behaviour and calming signals, etc. He may then understand more of what the dogs are communicating. Your Zig indicated quite classically what he wanted and was forced to do otherwise, not a good idea. Both dogs were put into a situation where their fight/flight instinct could very easily have been 'fight' due to their inability to get away, and with the instructor in the middle???? Not a position I would want to find myself in I can assure you.
It would be interesting to know what the other dog is like when he is not around such a large group of dogs and not being 'corrected', he may be very stressed and acting out as a result of the environment he finds himself in. Lots of praise for doing the right thing IMO, far outweighs lots of corrections for the wrong.
