Paul and Jeanette Gathercole said they allowed their daughter, Laura, 15, to hold a celebration at their home in Colchester, Essex, on Saturday to mark the start of her GCSE year.
The couple, who described themselves as "old fashioned parents", supervised proceedings and even marked off no-go areas, but several teenagers turned up uninvited after Laura publicised the party on Facebook. The party got out of control and 17-year-old Jay Whiston was stabbed to death outside the house.
Mrs Gathercole said: "We took every precaution we possibly could to make sure it was a safe party. We are quite old-fashioned parents, quite fuddy-duddy. We really did do everything by the book: the party started at 7.30pm and we had soft drink and food there.
"We were supervising it the whole time and patrolled the conservatory and kitchen and the parts of the garden where the children were allowed to go.
"The rest of the house was marked off with hazard tape; we put tape around the slabs in the garden so no one would hurt themselves."
Mr Gathercole, who works at a car dealership in Chelmsford, said that at 9.30pm they stopped one youth whom Laura did not know from entering. It is not clear what caused the incident, but within 20 minutes Jay was killed by a single stab wound to the stomach.
Mr Gathercole said: "We did not see what happened as the first we knew of it was when the paramedics arrived."
The family said about 100 teenagers attended the party, which Laura organised through the social networking website. She said: "They were all friends. I had a private page which could only be accessed by invited people: it was not a public Facebook page. The guests were allowed to bring a 'plus one' but they had to ask first."
Laura said she did not know Jay, who lived in Clacton, but said he had come with a friend of hers. She added: "There were about three or four people who I did not recognise at the party so I asked who they were: I checked with them all."
She said most of the invitations had gone to fellow pupils at Colchester County High School for Girls, which is ranked among the country's leading schools for GCSE and A-level results.
Laura also sent notes to neighbours warning them about the party and apologising in advance for any noise.
Mrs Gathercole said her family had been left feeling numb following the killing. "We are devastated and our thoughts are with Jay's family. We would like to express our sincere condolences to them," she said.
A 17-year-old boy from Colchester was arrested yesterday on suspicion of murder, the sixth teenager to be questioned.
Police also said another 17 year-old was being questioned on suspicion of murder. He had been arrested on suspicion of assault. A 17-year-old boy and two 18 year-olds were also being questioned, and a 17-year-old boy was released without charge. Det Chief Insp Simon Parkes, of Essex Police, said: "We're slowly piecing together the sequence of events that led to this boy's death."
