Supervised play in a puppy preschool is not the same thing as taking your puppy to the off leash area which you should not do under NO circumstances.
However, we all want puppies who are social with people but a lot of owners also want their dogs to be social with other dogs and go (at some stage) to the dog park. If you intend to never do this that is fine. But if you do and every dog will occasionally meet other dogs, you might want to consider safe puppy to puppy socialization for your puppy.
If they do not learn this as puppies they probably never will. Dogs have a so called finite socialization period which means what they have not experienced at a young age might cause them to go into fight or flight mode.
Like any other aspect of puppy socialization people of all ages, ethnicity, mobility, sounds, surfaces etc etc dog to dog socialization needs to be done in an appropriate and positive way.
Better no off leash interaction then inappropriate ‘free for all play’. Puppy to puppy socialization has to be only two puppies at any one time, of similar play style and confidence level and it has to be managed extremely carefully.
On leash interaction is not the same as off leash. I do not know about you but if I am walking my dogs I really do not want them to interact and play with every dog they meet. I want them to walk nicely on the leash, without pulling, and paying attention to me.
I know as a puppy it is cute but unfortunately not for long and you are setting your puppy up to fail. You also create the wrong expectation.
You are also potentially making your dog leash reactive. If dogs meet off leash they will do that in a circular motion, going for the bum sniff. On leash we make them meet head on which is confrontational in dog body language. If you make your puppy meet other dogs head on too many times even the best tempered dog can get scared and develop unwanted behaviours such as lunging, barking or even snapping.
Yes puppies have to and should play but it should be off leash. And it should be well moderated and supervised.
And again, the dog park is not a place to socialise puppies it is a place for well socialised and resilient dogs.