Ellen Cate, 7, has her math problems corrected by her teacher and mother Janine Cate, as her sister Claire, 13, does her own classwork, at the kitchen table in the family's home in Santa Clara, Calif. The Cates home school their children. An appellate court in southern California has ruled that parents who home school their children must obtain teacher's certificates.

SAN JOSE, Calif. — Henry Cate is a software engineer and blogger. And a teacher.

His students? His three daughters, ages 7, 11 and 13. Their classroom is the family's Santa Clara home. And, he and his fellow teacher and wife, Janine, say it's worked out beautifully.

But if a new appellate court ruling sticks, the Cates will have to go back to school to get a teaching credential if they want to continue home schooling their kids.

A three-member panel in Los Angeles ruled unanimously two weeks ago that parents who home school their children must have such a credential. Although the ruling will likely be put on hold during an appeal to the state Supreme Court, it could put a damper on the increasingly popular phenomenon of parents keeping their kids out of schools to teach them themselves.

The ruling has sent ripples of fear and confusion throughout California's vast and diverse home schooling community. If the decision is upheld, California would become the only state in the nation to require parents who educate their children at home to have a teaching credential.

The ruling could "essentially wipe out our home school program," said Whitney Holton of the Campbell Union School District, which has about two dozen children enrolled in its home schooling program this year.

The court case centers on Mary and Philip Long, a Southern California couple who home schooled their eight children through the Sunland Christian School in Sylmar. The family came to the attention of Los Angeles County social workers when one of the children claimed the father was physically abusive. The workers learned that all eight children in the family were home schooled, and an attorney representing the two youngest children asked the juvenile court to order that they be enrolled in public or private school as a way to protect their well-being.

Home school advocates blasted the Feb. 28 decision, written by Justice H. Walter Croskey of the 2nd District Court of Appeal.

"Instead of making a ruling that affected two children, he made a ruling that affects tens of thousands of children," said Debbie Schwarzer, a Los Altos attorney who home schools her two children and is active in the Homeschool Association of California. "What the judge has done is send us back to the Dark Ages."

As a published decision, the appeals court ruling sets precedent throughout California, although in the short term it handed down orders only in the Los Angeles case and is unlikely to force other school districts to make immediate changes to their policies. Other appellate courts around the state could still weigh in with differing opinions, and the California Supreme Court would have the last word — unless the Legislature decides to act.

"The ability to home school freely in California should not depend upon one family in a closed-door proceeding," said the Home School Legal Defense Association, which plans to supporting the Los Angeles family's appeal to the state's highest court.

Exact statistics on how many students are home schooled in California are hard to compile, but advocates put the number at between 150,000 and 200,000. The state Department of Education uses a more conservative figure of 50,000 to 100,000.

Some parents home school for religious reasons, but others turn to home schooling out of concerns over campus safety, peer pressure and the increasing focus on standardized tests. Others believe they can offer their children an education superior to what public and even private schools provide.

There are various ways to home school in California. Parents can file paperwork with the state establishing their home as a small private school, hire a credentialed tutor to educate their children, or enroll their children in independent study programs that are overseen by either charter schools or local school districts.

The school districts provide oversight. In Campbell, for example, two credentialed teachers help parents plan lessons, go over course work, provide instructional materials, guide curriculum and administer annual standardized tests to home schooled children.

Once largely the province of the radical left and conservative Christians, home schooling has enjoyed rapid growth in recent years and in some circles in Silicon Valley is considered mainstream.

The Cates started home schooling when their oldest daughter began kindergarten.

"A lot of kids go off to public school wanting to learn, and before you know it they are bored and rolling their eyes," said Henry Cate, who says home schooling is possible because he has flexible work hours.

Cate is not worried about the court case.

"The U.S. Supreme Court has said again and again that parents can home school their children," he said.