Questions are being asked about the Californian private education system after it was revealed the home where two parents allegedly tortured their 13 children also functioned as a school which had never been inspected by officials.

David Turpin and his wife Louise Ann Turpin have been charged with nine counts of torture and 10 counts of child endangerment after the siblings were found shackled to their beds. It has since been revealed that the 57-year-old father was listed as the principal of Sandcastle Day School as he home-schooled the children at the house in Perris.

According to its listing on the Californian department of education website, the private home school had a total of six pupils, one in each of the fifth, sixth, eighth, ninth, 10th and 12th grades, and was first opened in March 2011.

The abuse of the children was uncovered after a 17-year-old girl managed to escape and alerted emergency services. When officers arrived at the home, several of the children were found shackled to their beds with chains and padlocks in "dark and foul-smelling surroundings".

Authorities believed they had found 12 children inside the house, but were "shocked" to discover seven of them were actually extremely malnourished adults aged between 18 and 29.

Questions about how the private school inside the squalid home described as horrific was able to operate for so long have now been asked of officials.

Sherryll Kraizer, a child abuse prevention expert and founder of the Coalition for Children, told Reuters: "One of the things that was interesting was, he [Turpin] set up his own home school so the kids were accounted for and not really seen by anybody."

Private schools in California are not licensed by the state education department and therefore there are no official agencies which inspects or monitors them.

The schools are only required to file an affidavit with the state which lists the number of students, staff members and other administrative information every year.

The only requirement private school teachers need to satisfy is to be "persons capable of teaching" and have no felony convictions.

However, full-time private schools must be subject to inspections by the local fire department. Neither the local county fire chiefs nor the Perris fire department have commented on whether the Turpins' home was ever inspected.

Bill Ainsworth, spokesperson for the California department of education, said they are "sickened" by the discovery of the children and have launched an investigation.

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David Allen Turpin and Louise Anna Turpin in police mug shots after allegedly holding their 13 children captive in Perris, California Police handout

Relatives described their shock at the discovery of the children inside the home, telling how the family were seemingly "living the perfect life".

Louise Turpin's sister, Teresa Robinette, told NBC News from Knoxville, Tennessee: "She would tell us they went to Disneyland all the time. They would go to Vegas."

However, neighbour Kimberly Milligan, 50, said others spotted signs of possible abuse in the home. She said she had only ever seen the infant in the mother's arms and three other children since the family moved in two years ago.

"Why don't we ever see the kids?" Milligan queried. "In hindsight, we would have never thought this. But there were red flags. You never don't hear or see nine kids."

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Media gather in front of 160 Muir Woods Road from where authorities rescued 13 malnourished children held captive by their parents in Perris Getty