Almost one of every four K-12 students in Sonoma County attends a charter school.

That figure has practically doubled in just two years, and it's likely to keep growing for a couple of reasons. First, charter schools are popular with parents. After some initial skepticism, they've also gained favor with school officials.

Districts around the state are using charters as a defense against budget cuts. For some districts in Sonoma County, charters are a tool to combat enrollment declines.

The charter strategy, described in a recent article by Staff Writer Kerry Benefield, is generally sound — for now. State and federal laws create some financial incentives for charter schools, magnet programs lure students and their parents, and open enrollment eliminates obstacles to switching schools, especially if it involves moving from one school district to another.

As Steve Herrington, the county's school superintendent, said, "They are trying to use the system to their advantage."

Indeed, candidates for every sort of public office preached innovation during this year's campaign. School officials are practicing just that by making full use of charter schools.

When the first charters opened in the early 1990s, they tended to be started by teachers or parents, often dedicated to a unique program or approach. A few private companies formed to run charter schools. Today, in Sonoma County anyway, most of the new charters are existing campuses converted by the school districts that already ran them.

Three of the four schools in Petaluma's Old Adobe District are now charters. So are five of the eight in the Rincon Valley School District, and the district is making plans for a second charter middle school. Santa Rosa City Schools, meanwhile, plans to open an English-Spanish immersion school, the district's sixth charter program.

In addition to securing some added funding, charter schools are freed from some of the regulations and mandates governing other public schools. There's another important benefit: greater accountability. Charters can be revoked if schools don't deliver academically.

However, there isn't an endless reservoir of money for new charters.

"It's obvious in California the financial pie that is available to be divided for schools is not just a finite thing but a shrinking thing," retired teacher Gary Ravani told Benefield. "It's a zero sum game — if you are giving more somewhere else in a segregated situation, you are taking more from somewhere else. That is just unavoidable."

Moreover, opening more charters doesn't create a larger pool of students. Successful magnet programs don't erase the fact that public school enrollment in Sonoma County has declined nine years in a row.

Offering parents choices is good policy, and school districts deserve credit for developing programs such as Santa Rosa's ArtQuest and Rohnert Park's Tech High to attract more students. But, to use Ravani's phrase, it's still a zero-sum game.

The challenge facing Herrington and other officials is to find a cost-efficient model for educating Sonoma County students, whether they attend charter schools or not.