I have heard various things. It really all depends on the facility (IMHO). I go to the Denver Dumbs Friend League. They have ventilators and sterlize the floors after every class. they accept puppies from 8 weeks to ten for puppy preschool and they have to be at least 12 weeks for puppy kindergarten. All dogs have to have their initial shots for acceptance, and boosters for the Kindergarten classes.