See I have told you that Corgis are small dogs - definitely not medium size. If Peanut was put in with the toy dogs/other small dogs, then I doubt a similar problem would have existed - a small wayward puppy is not going to be as frightening. Organised puppy schools can be a reverse of what you want your dogs to get out of them because of adverse behaviour of other dogs and their owners/handlers. Better in these circumstances to take in methodology, and socialise your pup in an environment you have some control over.
If you want to return to the classes, you could ask the instructor if you could place Peanut among the other small dogs.