I've literally quoted from the article you've linked to.
Maybe you've misread the statement. It has been a challenge to "find out
more" about the role of schools in the spread of the virus. In spite of those challenges, they've seen enough to make the statement I've quoted. Which for some reason you've just pretended doesn't exist, twice. Fun times.
But that’s working on the assumption that all teenagers are running around meeting all their mates.
whereas we know for fact that they are in an indoor school environment, mixing with kids from other households, in badly ventilated buildings.
Yeah we have limited evidence on where else it comes from, and have to rely on hypotheses. What science is best at is testing and rejecting hypotheses, rather than proving theories. The hypothesis has been put forward many times that schools are obviously the place where it's spreading like mad, and the evidence has consistently ruled that out.
Sometimes there are super spreader events, just like sometimes there are super spreader events in all indoor areas. But it isn't somewhere that people are disproportionately likely to spread the virus, despite the fact kids are kids, and the schools are how they are. And the hypothesis for that with the most reliable evidence is that kids get infected less easily and spread it less easily, so more contacts lead to proportionately fewer transmissions.
That applies outside school too. Teenagers in general are socialising more than their grand parents in general. Likewise for uni students and their parents. It's not that all of them are doing it, but more are, and more often. Despite that, for much of this year kids have been infected no more often than their parents. Younger kids are usually infected less frequently, and teenagers for most of their time - and for most of the school year - were as likely to get infected as someone my age. Despite seeing many more people in and out of school on average.
That doesn't mean that there isn't some spread in schools - their evidence suggests the opposite. It does have an incremental effect on transmission. But their analysis is that schools are not the primary cause of the spread.