(CNN) Jennifer and Sarah Hart's 6-year-old daughter told a Minnesota public school teacher in 2010 that she had "owies" on her tummy and back after her mother hit her with her fist, leaving bruises.

Alarmed, the teacher alerted social services and police, who launched a criminal investigation. Six months later, Sarah Hart pleaded guilty to an assault charge and was sentenced to probation, county records show.

Just a week later, the Harts' six adopted children were pulled from their public schools in favor of a home-school setting, said Jill Johnson, a spokeswoman for the district, based in the small town of Alexandria.

They never returned to public schools.

The family moved, then moved again. Neighbors lodged new claims of maltreatment. Then, two weeks ago, as child protection workers in Washington state were trying to contact the Harts, their SUV plunged off a cliff in California, killing the parents and at least three children. The other three remain missing but are believed to have been inside the SUV. Mendocino County Sheriff Tom Allman, who is heading the investigation, says he believes the crash was intentional

The Harts' experience highlights what experts told CNN are classic signs of abusive parents isolating their children from other adults, including those who are mandated by law to report suspected abuse to authorities, such as teachers, doctors and police.

The combination of frequent moves, home-schooling and seclusion from neighbors, along with using food as a means of control -- all in play in the Hart case -- can signal the possibility of abuse, experts said.

"When you see families that are going to great extremes to keep their kids out of view," said David Finkelhor, the director of the Crimes Against Children Research Center , "that's a red flag."

Schools as a safety net

For children experiencing abuse at home, public or private schools can be a protection against mistreatment. Teachers, principals and other school personnel are "mandatory reporters," meaning laws require them to report suspected child maltreatment.

Adults can glean a lot from children who do nothing more than show up to school, said Nicol Stolar-Peterson , a licensed clinical social worker and child abuse expert.

A truck drives by the pullout where the SUV of Jennifer and Sarah Hart was recovered on March 29, 2018, off the Pacific Coast Highway in California.

"Are they wearing the same clothes for three days? Have they been fed? Are they always hungry when they get here? Do they eat breakfast and lunch and they're starving and they say that's the only two meals they get?" she said, ticking off the sort of details educators pick up on in a traditional classroom setting.

"There's a lot you can tell about kids just from them showing up to school," she said.

While home-schooling allows parents to offer some 1.7 million American children a personalized education -- often for religious, social or academic reasons -- it also reduces the number of interactions those children have with other adults, experts said. That can leave students without much protection if there are problems at home.

"It definitely reduces the amount of mandated reporters that have access to them. It reduces the amount of safety net for the kids to have other adults that they can go to," Stolar-Peterson said.

In another recent case, a California couple is accused of holding their 13 children captive for years in filthy conditions at their home. The Turpin family had registered the home as a private school , freeing it from the strictest government oversight.

"It's not something to make us intrinsically suspicious of home-schoolers," said Finkelhor, who is also a professor at the University of New Hampshire. "But it is a problem that we invite by having the easy home-school regulations we have."

Both Washington and Oregon, where the Harts lived, require families to notify school districts if their children are home-schooled, according to the Coalition for Responsible Home Education , a national organization of home-schooled alumni that pushes for reform in the movement. Thirty-nine states have such annual or one-time reporting rules

However, the Harts did not register as home-schoolers in the Clackamas Education Service District in Oregon, spokeswoman Kelsey Cardwell told CNN. Nor did they register in the La Center School District in Washington state, said Tammy Lichliter, an administrative assistant in that district.

And there is no federal database that closely tracks school enrollments that might have flagged this type of oversight, Finkelhor said.

"Even though we have mandatory school attendance, it's pretty easy for parents to say they're doing home-schooling, and they don't have to, in many places, submit their child to much in the way of examination and supervision by other people," he said.

New schools and new red flags

The 2010 abuse case involving Sarah Hart marked the second time school personnel had noticed something amiss with a child in that family.

Two years earlier, a 6-year-old Hart child was seen with a bruised left arm and told school staff that Jennifer Hart had struck the child's arm with a belt, according to Alexandria police records.

In an interview with Alexandria Police and Douglas County Social Services, Sarah and Jennifer Hart said the child fell down the stairs. They also said the child had faced "food issues" in public school.

"They say (the child) has been constantly going through food issues, where (the child) will steal people's food at school or eat out of garbage cans or off the floor," according to a police report.

Two months later, Markis, Hanna and Devonte Hart, the couple's three eldest children, left Washington Elementary School for a home-school setting, Alexandria public school enrollment records show.

The children returned to a different public school for the 2009-10 school year and to yet another school for the 2010-11 school year until they were again pulled after Sarah Hart's guilty plea, records show.

Using food as a weapon

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The problems with food apparently continued, reflecting a common method of mistreatment that can proliferate among parents intent on isolating their children, said Rachel Coleman, executive director of the Coalition for Responsible Home Education.

"Abusive parents wield food as a weapon," she said. "Home-schooling allows them to do so with sometimes-deadly efficiency."

A few weeks before the fatal crash, Devonte Hart started showing up at a neighbor's house in Woodland, Washington, "asking for food and saying that they were taking meals away from him due to punishment," neighbor Bruce DeKalb told HLN.

"It started out as one time a day and escalated up to three times a day, until a week went by and we decided that we needed to get professional help," he said.

DeKalb said he called Child Protective Services on March 23, three days before the fatal wreck, to report what he suspected was child abuse.

Begging help from strangers

In November, shortly after the family moved to Woodland, the father of a woman who lived next door to the Harts called 911 with a similar disturbing report

"The other night, a little girl jumped out of the second-story window on the roof and then down to the ground and then ran to my daughter and, this is like 2 in the morning, begging them to help her," according to the call, which was released by the Clark County Regional Emergency Services Agency.

He said the girl cried and begged his daughter not to let her parents know she was there. But they were notified, and a parent came over.

"Then she (the parent) had all four of the kids come back later and say everything was okay, and they were all standing at attention, like they were all scared to death," the man told dispatch. "And I think there's something very serious going on there."

That kind of coercive control, particularly over food and finances, often is a key factor in domestic violence cases, Stolar-Peterson said.

"If I'm a (domestic) batterer, I want to isolate you, I want to isolate you from the rest of the world," she said. "How can I isolate you? I can keep you home, I can control finances, I can control your food, I can control every aspect of your life. That's what batterers do, typically. Everything is about coercive control."