So, if schools must open, the data reported [NYTimes article] here would argue you use school space to accommodate elementary students and bring back middle and high school students in small groups periodically expecting them to learn more remotely using online techniques.
The data come from a study in South Korea indicating that kids 10 and under are far less likely to transmit COVID than adults, but children over 10 approach the transmission rate among adults. This difference conveniently matches differences in independence and the social need for adult oversight. Adequate social distancing is not possible in many schools so it would be possible to take advantage of this age difference to use existing space more effectively.
As the researchers tend to say, more research is necessary. While the researchers do say this, educators, parents, and politicians are running out of time to make decisions so these data may help those seeking guidance.