Some schools have been opened for months. Here's what they learned.

Some of what they learned is consistent with what many scientists have been touting—that masking, ventilation, distancing and regular testing when possible are effective ways to reduce transmission of Covid-19 in schools. Other once-lauded tactics, such as daily temperature checks and deep cleaning of surfaces, have become lower priorities.

They also have learned that teachers, not their students, are likely the primary transmitters of the virus in grade schools, that children are likely most at risk of infection during lunch time, and that tools such as portable air cleaners and carbon-dioxide monitors can help.

Some findings run counter to conventional wisdom. Hybrid learning, for example, where students split time between the classroom and home, may actually be riskier in some cases than full-time in-person classes because children are potentially exposed to more people. And it is likely fine for students to be 3 feet apart from each other instead of 6 feet, something the Centers for Disease Control and Prevention noted when it changed its guidelines on Friday.