This is what they were talking about, exposure __does not require__ you get sick. You can still be exposed, but you don't need to let the concentrations of pathogens to build up in stale air over the course of a school day.
The results aren't in yet, and of course will be subject to reproduction/duplicatation, but I suspect that's what we're seeing here. People (children) are still getting sick, just less so, presumably because they're getting exposed to lower concentrations of pathogens. The article doesn't make it clear (or I missed it) if the reduction is in severity (time spent recovering from being sick), frequency (number of times one gets sick), or a combination thereof. If I'm right, I'd expect it to be a reduction stemming from both reduced frequency/severity.
Most low-level infections probably go unnoticed, and won't show up in the study at all.
Some days the kids are just a bit off, and taking their temperature shows slightly elevated temperature (37.2°C in ear). I'm pretty sure they are fighting some mild infection, but they aren't really ill.
The other poster made a statement about the strength of the immune system. I don't think there's much evidence that exposure to virus A ends up contributing to a robust response to virus Z. There's some benefit if the infections are somewhat related, but not if they are dissimilar. So sure, exposure to a particular infection is likely to make your immune system more responsive to that infection, but it's probably not reasonable to say that it has gotten stronger.
There is a lot of evidence that exposure to Covid induces long term permanent damage to tissue and the ability of the immune system to fight off other infections.
> There’s a lot of evidence that exposure is required for immunity
Think about - immune system evolved over millions of years, is it plausible that it needed wild tribes of hunter gatherers to huddle in a nearly airtight box for 8 hours and infect each other to work
In the extreme there is evidence that no exposure to pathogens might weaken immunity, but that is not going to be a problem unless someone is living in a bubble or on a space station.
In your hypothetical of it only costing a few dollars a year the economic savings just by not having children be sick even once would easily outweigh any downsides. Also, we already have this for many diseases. They’re proven and they work just fine. Nobody reasonable is advocating you avoid the flu shot just because “everyone now has the same immunities”.
