This should be a good time for San Francisco's public schools. Test scores are rising, the district has earned national praise, and voters have approved a measure that will add millions of dollars to the school budget each year. Instead, the schools are at a crisis point, and the main cause is beyond their control: High housing prices are spurring many families to leave the city. As a result, enrollment is declining, schools are closing and officials are concerned the turmoil will reinforce negative perceptions that have helped give San Francisco the state's highest percentage of children in private schools,...