Thanks so much for your time. Your obedience instructor needs to go back to school. Feel free to tell her I said this. Your dog kills cats because of prey drive, not aggression. Aggression has to do with protection ones self.
Prey drive has to do wit chasing and killing things that run away.
You correct this by teaching the dog that cats are no longer prey items. This is done with a shock collar and obedience training. When you use a shock collar for this the intensity of the shock is as high as it can go. This falls under the category of one or two good corrections are better than 1000 nagging corrections. Take the dog out on a long line in an area where there are cats.