BigDog, you should first be proud that your dog has such a solid temperament. To get kicked and to shake it off and act normal afterwards is a great tribute to the German shepherd breed.
Getting angry at the guy is inevitable, but it doesn't help the situation. You did the right thing in not escalating it. When something like this happens to one of our dogs, we don't then want to make our dogs think it's something to be upset about - and that's exactly what we do when we start yelling and cussing and getting all worked up. That guy evidently had a problem and I feel sorry for his puppy more than anything. If he's willing to kick your dog, imagine what that pup is probably going through on a daily basis.
If I were you, I'd contact the trainer before your next class and talk to him. Find out what exactly is being done about the situation. It makes no sense to go back into a class that is set up for off-leash socialization if there's a person in the class who won't allow dogs to socialize with his dog. The pup did the right thing - submitting to the adult dog - and the owner doesn't seem to have a clue. If your dog was really pestering the puppy and the pup couldn't play because of it, then it's your responsibility to move your dog to another area. But regardless, unless there was an attack going on, kicking your dog was absolutely wrong.
I'm kind of surprised that they have adults and puppies in the same socialization group. Not all adult dogs are good with pups and not all pups submit well to adults (I have one of those). It seems that the class is almost asking for problems especially if they haven't carefully assessed the dominance/submission level of every dog.
Even in our puppy classes here, we have separate areas for the more dominant pushy dogs and the more submissive dogs so that no one gets overwhelmed.
Good luck, and I'm really glad to hear that your shepherd is such a nice ambassador for the breed!
Melanie and the gang in Alaska