Based on Walter’s observations, it’s reasonable to conclude that the students in his class gained activity immunity. It strongly suggests that the children who didn’t catch chickenpox avoided it because they had already been exposed to it through vaccination or an unplanned infection.
Remember, active immunity is when a person’s body actively develops immunity to the disease after being exposed to the full disease or an attenuated form of it via vaccination. In America, the chickenpox vaccine became a part of the children’s vaccination program in 1995. While it is possible that the children in Walter’s class didn’t catch chickenpox because of a vaccine, the term “previously suffered” indicates that they didn’t catch chickenpox because they already caught it naturally by organically interacting with other people.
Although, for parents who don’t want to give their children the chickenpox vaccine but want them to acquire early immunity to the disease, there’s something called pox parties. This is when parents intentionally expose their children to other children with chickenpox so that their children will catch it as well.
The question doesn’t completely rule out the possibility that the children avoided chickenpox during the outbreak because they caught it during a pox party. But even if the children caught it from a pox party—a prefabricated environment—it would still likely be considered natural immunity because the antibodies were acquired via exposure to the actual disease and not the diminished form in the vaccine.