They haven't been closed in most places for well over a year. And these kids still co-mingled. It's not like they just hung out alone in their rooms.

You aren't wrong. But, the question is how many viral seasons they have been open. You don't expect 100% spread in a single season.

We're also not seeing sars-cov2 spiking to high levels now along with RSV/et. al. This further suggests restrictions being lifted doesn't adequately explain all of the apparent phenomena.

Are we not, though? Covid hasn't gone away, and it seems to grow every season. Deaths are down, so that is very very good.

That said, I did not take that to mean that restrictions would be the only active component. I don't see any reason to think we can isolate this down to just a single prime mover.

Deaths from COVID-19 are way down because at this point almost everyone has already been infected at least once. The patients who were going to die from it are mostly already dead. Those deaths were tragic, but for the rest of us the combination of vaccination, natural immunity, and general health status makes it no longer a serious concern (or at least not much more serious than other endemic respiratory viruses).

Deaths are down because we learned how to treat the critical cases.