Because most of the research done with SARS-CoV-2 shows scant reduction in transmission with school closures. Most of the counterexamples ignore confounds. "We opened schools and transmission took off" should be "we opened schools, bars, religious buildings, restaurants, etc., etc., and transmission took off." One is presented as an observation for keeping schools closed; the other makes it unclear how to apportion responsibility, given what we know about how the virus affects different age groups.
All the "close schools to prevent transmission" relies on data from other diseases.
We only accept those results because COVID is just like the flu, I guess. Oh, wait, we look down on eedjits who say that. So difficult to straddle that 3'-high razor blade while doing that heavy lifting.
I think those under 10 should go back to in-person instruction. Put masks on the teachers (they won't stay on the kids, and most mask orders don't cover kids under 10). Few cases are shown to result from kids, kids that young get infected at lower rates even with all their transmission-friendly practices. It's not risk free, to be sure. But it's not even "moderate" risk. Plus that's the age where in-person instruction has the biggest payoff, when distance-learning is truly difficult, when kids need to be helped the most when it comes to abuse and malnutrition. Even just K-2 would be a big step to helping kids with maximum payoff and minimum risk.
Over 10? Iffier. The kids are increasingly more adult in their biology until they're about 16. Then their biology is pretty much adult, even if their brains aren't done maturing.