FROM the outside, their house looked like any other in the street. Neat and tidy. Sure, the family seemed peculiar, but the only thing that was really odd to neighbours was that the children were so rarely seen.

But we now know that the house’s ordinary exterior was hiding horrors within. Authorities alleged this week that David and Louise Turpin tortured their 13 children inside their various homes for years.

The kids were shackled to their beds by chains and padlocks for months at a time as a prolonged punishment. Forbidden from using the toilet, they were forced to relieve themselves where they lay. The only thing they were allowed to do while tied up was write in their journals.

They could shower only once a year, and were restricted to one meal per day. The kids were prevented from socialising with other children, and were not allowed to play with toys.

So, how did this horrific abuse manage to continue for years undetected by neighbours or the authorities?

The answer lies in the fact that the mistreatment of these children was hidden behind the veil of homeschooling.

The children, aged 2 to 29, were taught at the family home in Perris, about 100km southeast of Los Angeles, and Mr Turpin officially registered a private school, the Sandcastle Day School, at the address in 2011. The California Department of Education lists Mr Turpin as the school’s “principal” on its website. However, authorities said evidence suggested that only his children were taught there.

Education officials never inspected the so-called school and there is actually no legal requirement that they do so because private schools in California are totally unregulated. Not only is there no mechanism to check on the welfare of the students, there is no agency that tests to see that the children have met basic academic benchmarks.

The education department said it was “sickened by this tragedy” but stressed that it had no legal power to “approve, monitor, inspect or oversee” the school.

Little is known yet about what — if anything — the children were taught while under their parents’ thumb.

Mr Turpins parents told ABC News that the homeschooling was “very strict” and that they were expected to memorise long Bible passages.

The 13 children were ordered to write in journals — even while they were tied up — and officers have seized “hundreds” of them for evidence.

Speaking to reporters on Thursday, Riverside County district attorney Mike Hestrin said the children were so badly malnourished and abused that many of them were cognitively impaired.

None of the children had ever seen a dentist and they had not had a medical check-up in four years.

Their isolation left them with clear gaps in their understanding of the world.

“The children lacked a basic knowledge of life; many of the children didn’t even know what a police officer was,” Mr Hestrin said.

“A 17-year-old, when asked if there was medication or pills in the home, didn’t know what medication or pills were.”

While there is no evidence to suggest that children taught at home are more likely to be abused, homeschooling allows abuse to carry on undetected, away from the eyes of teachers and authorities.

The Coalition for Responsible Home Education (CRHE) keeps a database of homeschool abuse cases and it has as detected 380 incidents of severe or fatal child neglect since 2000.

“We would not say abuse is more common among home-schoolers, but when it does occur, there are fewer safeguards, less to stop it from spinning out of control,” CRHE executive director Rachel Coleman told Reuters.

The Turpin parents used many of the cruel techniques the group has observed in other cases of homeschool abuse, such as food deprivation and children being locked up.

“This case fits a pattern we’ve seen of isolation and imprisonment in abusive homeschooling situations,” Ms Coleman said in a separate statement.

“We know that many homeschooling parents provide their children with a safe and child-centred home environment.

“Unfortunately, current law provides nothing to stop families like the Turpins from using homeschooling to isolate and imprison their children.”

Research suggests that homeschooling is a way for abusive parents to maintain control over their children.

A 2014 University of Wisconsin study examined 28 cases of extreme child abuse, including 10 children who had died as a result.

Just under half of the children studied had been removed from school for homeschooling. A further third had never been enrolled in school.

“This ‘homeschooling’ appears to have been designed to further isolate the child and typically occurred after closure of a previously opened CPS [Child Protective Services] case,” the study stated.

“Review of these cases found no true educational efforts were provided to the homeschooled children.

“Their isolation was accompanied by an escalation of physically abusive events.”

The study concluded that parents or caretakers carried out the abuse in an effort to “crush the child’s spirit and humanity”.

The CRHE argues that incidents like the Turpins’ would be less likely if officials were required to make contact with homeschools annually, and that background checks were performed on the parents who run the schools.

Mr and Mrs Turpin have each been charged with 12 counts of torture, seven counts of abuse of a dependent adult, six counts of child abuse and 12 counts of false imprisonment. Mr Turpin faces an additional charge of performing a lewd act on a child.

They have pleaded not guilty to all charges and are due back in court in February.