The main risk is transmission inside school buildings, but there are ways to reduce the chance of a big outbreak. Germany and Norway reopened schools with stringent precautions such as distancing and masking. But when Israel reopened schools, large outbreaks followed.
In my home state, Connecticut, K-12 students will be required to wear masks. Many districts are attempting to change the structure of the week to make classrooms less crowded. New York City and other districts are contemplating staggered schedules, in which students would spend some days in the classroom and others at home.
The use of hand-washing stations will be routine. Desks will be farther apart. Teachers should be given protective equipment. Colleges are using pooled testing of students and teachers to reduce risk, and this practice could be adopted widely. These measures are costly, and districts will need financial and technical support. Reopening schools will also require keeping outbreaks under control. It would be more difficult to reopen safely in Florida right now than Massachusetts. Prevalence matters.