I think it is very dangerous to keep this dog in a group class. The club risks much bigger problems that not being PC about a breed ban, if the dog attacks another dog. The fact that it already went for the cocker is a sign that it simply cannot be in a general class.
Please do not risk other people's dogs in order to help a dog that needs one on one work. The club can surely offer a home training class if it wants to work with the dog or wants you to work with it -- but this is a situation that is NOT a basic obedience training situation and such training would still not make it acceptable to be in a class with other dogs. I have seen exactly such dogs being worked by Dee Ganley, Tara and Lisa, and it really, truly requires specialist handling with NO other dogs there, and many months of training in most cases. many will always need to be walked on a muzzle, some even wearing the calming cap. It really horrifies me that the club expects the dog to be including in ageneral class. Please at least have someone in the club ring a certified APDT trainer for advice before proceeding in this direction.
A waiver would NOT be good enough in a case where poor judgement could be proved. The cocker's owner making a statement would be enough to convince any judge that the club did not take adequate care amd is fully liable. Also, the fact that the dog was never professionally assessed before being allowed in the class. Simply signing a waiver doesn't release trainers or instructors from a provision of care towards clients.