The school also needs a macro plan. Let’s say you will keep the school open even if there are some cases: Is there a point where an outbreak is large enough that you would close the school? Again, guidelines are vague on this. The C.D.C. doesn’t make any concrete statements.
We might look to places that have had open schools for evidence of what worked. Many European countries have opened schools, largely successfully. They did so taking various approaches to closures. In Germany, classmates and teachers (but not the rest of the school) were isolated for two weeks after a reported case. Taiwan, apparently, planned to close schools for two or more cases but as of early this month had yet to face that. Israel, which has had probably the most fraught reopening, closed schools for every case. This has resulted in a very large number of school closures.
The evidence from other countries suggests schools could take a variety of approaches. My view is that the most important thing is that they are explicit about which approach they will take. Not just in broad strokes, but in detail. And more than that, in advance.