NPR:

An appellate court in California recently ruled that parents who home school their kids may be breaking the law. The decision requires parents to have filed paperwork to run their own private school, or to have enrolled their kids in a satellite school or to have credentialed tutors to do the teaching.
Luis Huerta, an assistant professor of education and public policy at Columbia University, says the decision could have massive implications not just for the nation’s home schoolers, but for privacy advocates and future Supreme Court decision-making.
It’s difficult to give a snapshot of the people who home school in the United States, Huerta says. “It’s an elusive number, and it’s very difficult to track them down,” he says. “If they chose to home school, they’ve chosen not to report to the state.” He says there are probably 1.2 million children taught at home in the United States, up from 600,000 in 1996, a doubling in a little more than 10 years.