Earlier quoted context omitted.
The point of upzoning is to allow for more housing to be built, not to literally force it upon unsuspecting property owners. If nobody wants to sell, nobody wants to sell. But eventually...
Houses in Atherton are already the size of apartment buildings. You could house 10-20 units in them without any change to the exterior. Change will happen, people would gladly pay a normal Bay Area house cost to get 1/10 of an Atherton house and live in that school district
School districts/boundaries being so unequal is at least partially the result of zoning policy that makes it harder for people of less financial means to move into affluent areas.
I saw a counterexample living in Munich, just in the sense that the differences between neighborhoods in apparent wealth were much less stark than in the US. Almost everywhere looked vaguely middling, very few places looked obviously rich or poor. And so it's no surprise that the differences between public schools was lesser too.