I think the authors of the study would tell you themselves that the study isn't comprehensive enough to figure out the exact threshold at which we should re-open schools. They'd really need to do a 50-state study for that or at least a larger scope than two states, because as they noted, the threshold was different between the two states they looked at.
It was and it continues to be a failure of our government throughout this entire pandemic that we aren't spending as much money as necessary to do studies like this but on a larger scale. We should have had this data six months ago. We have the underlying data and the money to do it. And to break it down more granularly by age as you're suggesting. It's a travesty that we haven't done it, but as just this thread alone demonstrates, the anti-science undercurrent in our country is dangerous.
What it does tell us is that schools do increase community spread except when virus prevalence is exceptionally low. Which was of course the obvious conclusion all along for anyone not treating this as a partisan or personal issue.
The discussion all along should have been "School is hugely important, so how do we protect students and teachers to mitigate as much risk as possible, and how do we get teachers vaccinated as quickly as possible?". Instead, it turned into "hurr durr schools don't spread Covid" idiocy.