It depends upon where you are. Currently, some of the big trainers are recommending starting at 10 weeks for puppy kindergarten. I have started that early. In reality, the risk of contracting something from healthy puppies in a class environment is small, overall. The only thing that I have ever brought home from a puppy class was kennel cough (one puppy had it, but was not showing symptoms yet) - I don't vaccinate for that, and it was my adult demo-dog that brought home the illness. None of the other puppies in the class contracted it, and as soon as the owner of the ill puppy knew that his dog had kc, he pulled her from class.
Now, my dog club has a rule that the dog cannot start until it has had it's second set of shots. They don't need the full course, as the third shot should not be given before 16 weeks (following the Dr. Jean Dodds protocol).