The numbers come from an HSPC report:
Obviously there are a number of different environments other than a school in which young kids could catch Covid, but does anybody here believe that anywhere other than school is the most likely source of most cases in this age group, and if so what is that source?“The risk of onward transmission from undetected asymptomatic cases within the school setting remains low”...
...It states that increases in the disease incidence among school-aged children, or the detection of an outbreak in a school, does not of itself indicate that transmission occurred within the school....
“Clusters of cases may be detected within a particular setting despite exposure and transmission having occurred elsewhere,” it adds.