...if just continued classes will help socialize her. First, some dogs on leash are just not good when their personal space is invaded. The dog feels trapped and can't do a proper dog/dog introduction on leash. A book called "Feisty Fido" can help with on leash dog aggression. Yet, some dogs really prefer just not to interact on leash with other dogs. I find my dog works better if I make class time "work time" and other times are play time. Just like you go to work and don't necessarily want to go out for cocktails with all your co-workers, you dog doesn't have to either. There's nothing wrong with protecting your dog's personal space in a class situation. Your dog might thank you for it. If you do it, your dog doesn't feel she has to. Keep the leash loose around other dogs and verbally tell other dog owners you dog requires some space. Also, during these moments, treat your dog for not reacting at all. Quite literally treat your dog for sitting there and doing nothing. That's exactly what you are looking for. There are a few other techniques that work as well for desensitization to dogs- especially if the problems are fear based. "The Cautious Canine" goes into those techniques.
Now, off-leash is a different story. If your dog is not well behaved off-leash in the presence of other dogs your work is harder. Agility is 'controlled chaos' sometimes. Dogs in training break from their owners and romp around. You don't want your dog picking a fight with the dogs that are waiting (if your dog is prone to that). So you would have to work extra hard on attention and think about the safety of the other dogs if you made an error. Poor socialization is something that can be remedied, but an adult dog is no longer a puppy and it takes time. Also, have your vet check your pup for any medical conditions. Agility puts stress on the body and small things can be magnified with the activity. It might make your dog grouchier than usual. Pain is often overlooked and dismissed as a cause for aggression, yet it is very real. Hope that helps a bit.
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Chelle and the rest of the crew including, but not limited to Kita (the shiba inu), Adi (rescue tabby cat), and all the reptiles