For many Bay Area school districts, persuading voters to pass parcel taxes in Tuesday's election isn't about selling new and innovative programs in these lean economic times -- it's about pleading to keep what they have. From Walnut Creek to Milpitas, superintendents and school boards in 17 districts are trying to maintain everything from music programs to school nurses to lower class sizes as they calculate budgets for next fall. Declining enrollment and unrealized monetary pledges from Sacramento have squeezed many districts in recent years. "We've been a high-performing district ... we have high expectations for our students," said Superintendent...