The Bay Area has a really interesting synergy of situations that all contribute to really terrible public schools. A big one is that even in the best school districts, the salaries do not match the cost of living, so teachers cannot afford to live in the communities they work. Another one is that, thanks to prop 13, low property taxes on long-held properties create a serious bottleneck to funding for the schools. Both of these serve to strangle off the quality of education, which in turn drives parents to private schools, landing us at the point where enrollment is dropping off and, as you say, lowering school funding as well.

I would argue that until the Bay Area housing crisis is resolved (probably in part by repealing prop 13), the private-school exodus will continue.