In my humble opinion 6 months is a bit late to start classes with dogs. I'll clarify that statement though. I believe all puppies benefit from puppy socialization classes and fun, informal behavioral training classes where they learn to sit, heel, down, stand, stay, come and a few fun tricks {Puppy kindergarden).
Now, just because a puppy learns to do these things, those behaviors really are not truely "learned" and generalized until a dog is much older- some dogs never learn this because their owners believe one 8 week class will teach a dog all it needs to know for the rest of it's life.
OK, so a 6 month old pup really should start attending beginner obedience classes (elementry school) that focus on positive reinforcement and solidification of behaviors learned in puppy classes.
A 1 year old dog can mentally handle the dog equivalent of Junior High School and depending on the maturity level of the dog and foundation skill sets established, an 18 month old dog can handle Senior High School.
Now, all training schools are different so it's very tough to break down what the class titles translate to for your dog, but I hope you get the idea. In my humble opinion, a dog needs to be in foundation obedience classes for at least the first 2 year's of it's life. Doesn't have to be weekly classes, but there should be a daily focus (5 to 10 minutes a day) proofing the dog in new situations and places. In my experience, taking weekly classes keeps me focused on what we need to work on and helps me trouble shoot if something isn't working.
Now, if you want to go beyond the basics and get your dog ready for formal obedience competition, that's a whole different level of training and in my opinion can not and should not be rushed, but it's a fairly long process, but a fun one.
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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