It’s every parent’s nightmare—a celebrity child starts at your kid’s school, and within days, despite all the assurances you have been given that the place has never been safer, an unbalanced person is arrested, having successfully defeated the expensive security systems and gained entry to the school’s lobby area.

This is the reality that parents of children at Thomas’, the exclusive Battersea school where Prince George started this week, are coming to terms with this weekend, after a 40-year old woman obsessed with the ‘perfect’ lifestyle of the Cambridges was arrested and detained for 24 hours after she was found to have successfully talked her way into the school.

Incredibly, it emerged that the woman had been warned off by police on Wednesday after being spotted hanging around the school, but came back on Thursday afternoon to try her luck again.

The second time around she was successful, gaining entry to the school building itself before being detained.

In fact, George was not in the building at the time as new children go home before lunch for their first few weeks.

Prince William did not comment on the security breach, only remarking that his son’s first days at school had been ‘interesting’ when questioned by a member of the public at an event this week.

“It’s unbelievable that she got inside,” said one parent with children at another high-profile London school (parents at Thomas’s are quite properly observing an omerta and refusing to speak to the press).

The parent speculated that the intruder had exploited the confusion that surrounds the identity of new parents at the beginning of term.

“Thomas’ is a big and busy school, with about 400 kids, so you can see it would be easier to get into a place like that than a smaller school,” the source said.

“But it’s still shocking. The security at all the private London pre-preps is mega. They never stop telling you about all the security protocols we have to follow – and then someone just talks their way in?”

The school intruder, who has been named as Louise Crawford Chantry, 40, a former holistic healer who recently suffered a mental breakdown, defeated a complex web of security precautions at the school.

Tens of thousands of pounds have been spent on an electronic entrance system, which controls reinforced glass doors. CCTV cameras connected to a facial-recognition system also cover every approach to the school, which can be locked down behind 7ft wrought-iron gates. A minimum of two undercover police officers are stationed outside the school at all times.

William and Kate, who is pregnant with her third child, are known to be keen to provide their children with as ‘normal’ a childhood as possible and would be reluctant to go to the intrusive lengths that one parent told the Daily Beast some billionaires do: “There was a rumor that Roman Abramovich had his security guards sitting in the classroom at Garden House school when his kids were there.”

At many private London schools, parents said, adults are not allowed into the school buildings unless accompanying their children, and there is a separate entrance for adults.

Chantry was not unknown to the police either. She has reportedly admitted racking up £1,000 in unpaid hotel bills and three assaults, including two against police officers.

Her mother, Rhona Crawford, 75, from Bath, Somerset, told The Sun: “'She wouldn't have done any harm to George. She just loves the royal family and loves kids. She's been under a lot of stress. I'm very sad this has happened, it's a horrible incident.”

She added that her daughter views the Cambridges as the perfect family and said: “People were in love with Diana when she was alive, this is similar. It's a fairytale. It's a fantasy.”

Her remarks have done little to quell unease among parents at the other exclusive schools that dot the city.

A terror attack on Friday morning in London has done little to help settle nerves.

One London private school parent who spoke to the Daily Beast on Friday, shortly after the a terror attack saw a spike in security concerns across the city, said: “You try not to think about it, but the kids are sitting ducks. If someone can get into Prince George’s school, it really does make you worry that they can get in anywhere.”