Nearly 1.1 million US students were educated at home last year - up 29 per cent on 1999 figures - and the rise is driven by parental dismay at the “environment” in schools, according to government findings released last week.

The booming numbers suggest that home-schooling is moving beyond a fringe of religious separatists and hippie dropouts into the mainstream, experts say.

Some 31 per cent of home-school pupils’ parents polled by the National Center for Education Statistics, an arm of the US education department, cited environmental concerns such as drugs and bullying in their decision to pull their kids from school.

But 30 per cent said they wanted to provide their own religious and moral instruction, reflecting the continued strong presence of Christian fundamentalists among home-schoolers. Sixteen per cent cited schools’ lax academic standards.

“The religious conservatives and educational radicals who invented home education in the 1970s have created a movement that’s hard-wired into American culture,” said Mitchell Stevens, educational psychology professor at New York university and author of Kingdom of children: Culture and controversy in the homeschooling movement.

“It’s now a legitimate choice and what’s remarkable is that it developed under the radar of the academic world, while professional educators are explicitly critical.”

Home-schoolers account for 2.2 per cent of five to 17-year-olds in the US but 20 per cent of privately-educated pupils, said Clive Belfield, associate director of the national center for the study of privatisation in education at Columbia Teachers college, America’s largest teacher-training institution.

It is hard to gauge their academic standing, he said. “The only ones that take standardised tests are the highly-motivated ones. The evidence says they do well, but there is such a big selection bias that you can’t set much store by it.

“Also, if you’re taking a test at home, conditions are very different to 200 kids in a gym sweating through a three-hour exam.”

Monitoring of these students is also patchy. Twenty-nine per cent of America’s 50 states expect home-educating parents to notify authorities, administer standardised tests and file academic progress reports, according to the Home School Legal Defense Association. But compliance is often voluntary and enforcement is lax, Mr Belfield said.

California, America’s most populous state, requires only notification, while nine states - including the second-most populous, Texas - have no regulations at all.

The US government’s definition of home-school students includes those attending regular school part-time. But 82 per cent of those counted were exclusively taught at home.

Mr Stevens said home-schoolers typically tap extensive support networks and social outlets in their local communities and become adept at cultivating relationships with people of all ages, whereas school pupils mainly interact with their peers.

However, Mr Belfield said that home-educated children could be socially stunted.