Mine all caught it right at the start of the summer holidays. Was when we got back to school we discovered half the kids in school (if not more) also caught it! Thing is with viruses like slapped cheek is that the incubation period is before any symptoms. As soon as rash comes, it is no longer contagious. So the spread cannot be predicted or prevented. The whole time my kids have been in school, the only thing we have ever been notified about is head lice. Even when a significant portion of the kids got chicken pox, we only knew from other parents.