The first cases appeared unrelated: a rash from a brush with poison oak, perhaps, or maybe an allergic reaction to insect bites. But as more children at the Stinson Beach Preschool kept itching, the teachers realized something else was going on: the school had an outbreak of scabies, a skin infection caused by microscopic, slow-moving itch mites burrowing under the skin. First identified by Aristotle as “lice in the flesh,” scabies is one of the…