Sue Dickinson, from Launceston, Cornwall, is suing Cornwall County Council following the murder of her daughter, Caroline, who was asphyxiated while staying in Brittany in 1996 as part of a school trip. If Mrs Dickinson is successful in her claim she will receive pounds 22,578 in damages.
On the first day of the civil claim, the High Court in Plymouth was told that the two external doors to the hostel were unlocked on the night Caroline died.
Caroline was murdered in a bedroom, which she was sharing with other four girls, by an unidentified intruder while at the hostel in Pleine Fougeres on 18 July 1996. She was among a party of 40 pupils, aged between 10 and 14, supervised by teachers from Launceston Community College.
Stephen Killalea, representing Mrs Dickinson, told the court that the teenager's bedroom was on the first floor of the hostel, directly above the front door.
Speaking publicly for the first time about her daughter's murder, Mrs Dickinson said in a statement given to the court that she was "horrified" to hear about the lack of security.
Before the outing Mrs Dickinson met Liz Barker, a teacher who was responsible for the trip. "[Mrs Barker] said that it was small and friendly and she knew the people who ran the hostel from her previous visits," Mrs Dickinson said.
"Nothing was asked about the security or how the hostel had been chosen. I simply assumed, as I think most people did, that the accommodation would be suitable for school groups and secure."
Mrs Dickinson said that Caroline, who was studying French and was "very independent and bright", had paid for part of the holiday herself.
She left on a coach on 14 July and later that day telephoned home, describing the hostel as "crappy".
Mrs Dickinson said: "I feel that they [the school] failed to carry out any appropriate checks to ensure the suitability of the accommodation for a group of schoolchildren. As a direct result an intruder was able to walk into the accommodation without any challenge and murder my daughter.
"I cannot understand why young children were allowed to sleep in a totally unsecured and unlocked building with unlocked bedrooms.
"Even if the building could not be locked at night, which I do not accept, it beggars belief that the bedroom doors were not locked."
Mrs Barker, who fainted in court after giving evidence, described in a statement how Caroline's body was discovered on the morning of 18 July. Two girls knocked on her door and asked if she could go and see Caroline "as they couldn't wake her and she was a funny colour".
Mrs Barker said the intruder could have "easily" climbed the outside of the building and entered through the bedroom window.
Timothy Kerr, for Cornwall County Council, said every aspect of the arrangements for the trip had been completed competently and diligently.
Also in court was Mrs Dickinson's former husband, John, from Bodmin, Cornwall, who is not a party to the civil action.
The case continues.