You completely and utterly ignored the first of the two studies in the linked source.
Which would seem to indicate that open schools would provide exponentially more child-to-child contact, exacerbating the findings of this study.
Or in other words, children represented 0.94% of the sample, but 1.27% of the spread.
And every non-child infected less so. As you said, the lockdown was in place and schools were closed. Do you think the R value stays under 1 with no lockdown and open schools?
I would imagine that's because of prolonged immediate exposure in an enclosed environment. People are quite often seated much more closely to others and for longer periods of time in an office environment than even at home. Schools are more similar to the work environment, except more tightly packed and quite probably much more vocal, especially among the younger classes.