> absence of evidence is not evidence of absence

That applies to infection/transmission via schools in general. Precisely because we don't have decent testing and tracing, we have a large number of cases whose origin is unknown. Schools have to be considered as one possibility, especially considering the several cases where reopening schools has been strongly correlated with spikes in infections. The precise mechanism doesn't matter. It's a red herring. Whether it's kid to kid, parent to teacher, cafeteria workers, whatever, it seems to happen some of the time. Until we've clearly ruled out a cause for those unknowns, we have to consider it still operative.