Masks have been optional at school since the beginning of the school year here, with very few kids wearing them. My daughter wore hers for the first part of the year, until she was able to be vaccinated, and she was one of only 2 kids in her class of 20 who was doing so. We let her stop wearing hers after she was vaccinated, because the one other kid in her class stopped wearing his around the same time.
At the beginning of the school year, during Delta, cases at our school were CRAZY. There were times that over 25% of my daughter's class was out, major teacher shortages, etc. It's hard to say whether the transmission was occurring at school or at home, though, because our community cases were also crazy. Omicron didn't hit nearly as hard in our schools... I'm not sure whether that's because everyone had just had it, or because people had given up on testing.
As far as we know, our daughter never got it from school. I test her with a rapid test at any sign of sniffles/fatigue/etc., and she's also had several PCR tests, when she was sick enough to warrant a pediatrician visit.
(I had COVID back in 2020, when tests were only available through hospital or urgent care. We think my husband also had it, because he was the only one of us who was leaving the house. Our daughter never had signs, so we never had her tested... unsure whether she had it or not.)
