You need to work on your relationship with this dog off the agility field before taking him to such a high distraction area.
First I'd work on having him choose to come near you. Again, this work needs to be done away from all the sights and sounds of agility classes. Starting in your home and treating your dog for offering the behvior of being near you and working with you is where you need to begin. Grab some treats and walk around the house. More than likely, your dog will come looking for you at some point. Give your dog a treat. This will probably seem quite easy- good. That's kind of the point right now. Now, go to an area of the house or garage that you typically aren't with the dog. Make sure it's safe for the dog to be there and repeat the exercise. The dog will typically follow you at some point. Once you get the attention focused on you in this new enviornment, give a treat. If the place is very new to the dog, msot will grab the treat and take off sniffing again, fine walk the opposite direction. Most dogs will follow you when you walk away from them. As soon as your dog chooses to follow you, give treats and party. Progress this to the front yard (use a long line of about 20' or so tied to your waist). Same thing. Dog goes outside with you and when he's distracted by the outside walk away from him. When he follows treat lavishly. Then continue this in outdoor tennis courts, hocky rinks, softball fields, grocery store parking lots. Always make sure your dog is safe and use the long as "insurance" to make sure he's safe and not as a was of forcing hte issue to come to you. The goal here is to make yourself and your movement attractive for the dog to follow.
When you go back to class, use that same long line and DO NOT let this dog interact with the other class dogs until after class and only if the other students want to. Class is a place to work and not play. The reward for working in class can be play after. Crate your dog in class on th side lines of you need to enforce this. The FUN comes from you in class.
ALso, bring high value treats to class that the dog only gets for paying attention to you. Most BCs can be taught to enjoy a game of tug on a toy. That is a way for you to play with your dog and to reward your dog so the rest of the world is less appealing to explore.
BCs are smart. This training should go fairly fast. Just don't give up on the dog and be consistent. Watch out for what you are actually teaching this dog as well and be careful not to fall into the trap thinking because the dog is smart they automatically know what we want from them. It's our job to show them the game and the rules first and then show them the rules apply everywhere.
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Chelle and the rest of the crew including, but not limited to Kita and Taiko (the shiba inu wrestle maniacs), Adi (reserved and dignified tabby cat), and all 28 reptiles