To protect teachers, is the main reason. While one kid may be a less likely transmitter, soon all the kids in the classroom will have CoVid and the viral load in the classroom will go up. That's why they're cutting class sizes in many places (entire states).
In the US and UK, the average age of death from CoVid is lower than in France, Spain, Italy and China. Diabetes and obesity rates are higher. Lots of teachers are in their 50's and early 60's. The liability to the school districts if just one teacher dies or has lifelong health consequences is enormous in the US.
Of course, kids with yet undiagnosed underlying conditions would also die or have serious consequences (more lawsuits).
Further, some people will homeschool their kids rather than have them get CoVid (which surely, many will do when schools open - we closed schools in California before the virus had really got a foothold in any of them, it was mostly L.A. Marathon runners who ended up spreading it in LA, but also LAX and hospitals themselves were a main vector).
I posted a link yesterday - 20% of hospitalized CoVid patients got a severe case from having been previously hospitalized. It's normal in infectious disease for hospitals and clinics to be vectors. But in a second wave, it will be the schools.
Up to 20% of hospital patients with Covid-19 caught it at hospital
(the link again)
While very few children will have serious consequences, the fate of teachers will not be free of CoVid. My granddaughter's teacher, for example, has a child who was born with cancer, has since developed another cancer, and is severely immune-compromised (but has reached her 5th birthday - her first year of life was entirely in a hospital, everyone had to be masked and gowned to visit her). That teacher is only 36. She's also the main breadwinner in her family.
So we better get crackin' with hiring new teachers to replace the ones who will have to quit (or go onto some sort of disability plan) when school starts. One of my colleagues has a wife who is severely immune-compromised. They don't want to homeschool their two children, at all, but that's what they'll have to do in August if kids go back without masks. Even with masks, it's going to be a worry.
At any rate, I think schools will end up closing their doors and consolidating - as yes, just that many people will homeschool. So many are already on the verge of doing it anyway. The collapse of public education has many longterm consequences, because the shared social values (and the need for self-control in public) will be impacted.
Interesting times, indeed.