I am no longer updating this page. An expanded version is being published every Friday by noon as the Shoestring Covid Tracker.
Some of the CDC guidance on school reopening is below. It should be noted that the guidance is controversial; see this New Republic article for an excellent discussion. It is clear, though, that the CDC metric does a better job of separating different degrees of disease spread than the Massachusetts one does, especially in Hampshire County – see this discussion and the Northampton and Amherst graphs here.