Propagated by veterinarians, breeders and trainers as the single best way to prevent fear and insecurity rooted aggression and reactiveness, many a young pup after the second vaccination pulls his/her proud new parents into the training facility, where he/she experiences and learns to, under the blanket of socializing: rough-house, bully, be bullied, get her fun from the environment - not the owner, not experience safety if bullied or handled by too many people.

New puppy owners are so brainwashed by the 'impressionable time running out and need to pack as much stuff into the first 16-weeks of life", that they forget to pay attention to what the puppy really needs. The facts are that baby dogs are not all alike, because of genetics and imprinting (the very first few weeks of life). By the time a pup joins his new family, some behaviors are already established, some fears formed. That means that a generalized approach to socialization and early training is not working for a good number of puppies that are already fearful. For those, a puppy class increases the fear and can set the pooch up for lifelong behavior problems. The effect is opposite to what the well meaning owners intend and are made believe they achieve by enrolling the pooch in a puppy class.

A fearful pup needs to learn things, needs to be acclimatized and familiarized, but foremost needs to experience emotional safety, and I have yet to observe a class where that is taught or is experienced by the pup.
Rather than forcing the thin-skinned baby to be touched by human hands, handled by other people, and have other pups in her face, she should be allowed to experience what the world has to offer - the real world she lives in, not the training facility, in a safe enough distance so fear does not arise. That will build security and trust in her humans, and set her up for future training success.

The confident and playful pup first has to bond and learn to pay attention to his owner(s) before he should be given freedom to play within a group of puppies, or he will learn very early on that he doesn't have to listen; that the best fun happens away from the owners - not with them.

I instructed puppy classes and never liked them. I participated with a foster puppy in one of my friend's class and never should have - in hindsight I am convinced that it increased her fears and delayed her progress.
As a result I began to incorporate puppies into my beginners classes. All my classes accommodated individual needs of dog/owner already anyway, so it was easy to incorporate puppies into beginners.
The more I did that, the more I liked it. The pup was exposed, but in a more structured way - not a free-for-all bouncing time. He also met adults, not just other puppies, which resembles the real world. Off leash playtime groups were arranged by matching and balancing dog personalities - not age.

Puppies are impressionable. It is true that opportunities missed are difficult to make up for later in life. That doesn't mean we have to pack a lifetime of stuff into eight weeks, stress the puppy out by too much stimuli and too little rest and bonding, increase his fears, or let him get away
with bullying. Leadership and the human/dog bond always should have priority to the environment.