At what age should a puppy go to puppy school?

There is a bewildering amount of confusing and conflicting advice given by various breeders, pet stores, trainers and vets!
 
A medical argument from some breeders, pet stores, vets and trainers is that if you take a puppy too early to puppy school he could get a fatal disease, such as distemper or parvo virus. Therefore it is best to keep your puppy quarantined or isolated, as if in cotton wool, till he is four and a bit months old and has had all the required vaccinations.

The counter argument is from other breeders, pet stores, vets and trainers who take into consideration both the medical and behavioural risks involved. They look at the bigger picture and conclude that the latter risks far outweigh the former. By the time your puppy is four months old he is almost adolescent and many undesired habits may have formed and it is much too late for puppy school!