However, the friend told the hearing in Hartlepool that she did not think Elaine Swift had meant it.
The inquest had earlier heard claims that the 15-year-old had suffered years of bullying after donating bone marrow to her younger sister Christine when she was aged 11.
The family, originally from the Stirling area, moved to Hartlepool about 10 years ago.
"
It was nothing serious, I didn't believe she meant it
"
Elaine's school friend
Elaine died in Newcastle's Freeman Hospital in November 2001. She had undergone a liver transplant following her overdose.
The 15-year-old was studying at Dyke House Comprehensive School in Hartlepool after being transferred from another school in the town.
The inquest at Hartlepool Civic Centre has heard that she had become something of a local celebrity due to the publicity which followed her bone marrow donation.
Her father believes that this led to bullying which then became unbearable for his daughter.
'No pact'
One school friend, who has not been named, said she and Elaine had shared messages in a school exercise book.
On one occasion the friend wrote: "How are you doing, I'm bored so I thought I would let you know that I'm going to kill myself tonight."
She said Elaine had replied: "Hi, it's Elaine here, I'm killing myself tonight."
However, the girl denied that they had forged a suicide pact.
"
I did not feel that bullying was the issue and that there was much more underneath
"
Susan Davis
Social worker
"It was nothing serious, I didn't believe she meant it," she told the inquest.
Elaine had received 37 counselling sessions after being referred to specialists by her school.
Therapeutic social worker Susan Davis said she did not believe bullying was at the root of the teenager's emotional problems.
"I did not feel that bullying was the issue and that there was much more underneath. I felt there was a lot I did not know about," she said.
"She did seem emotionally vulnerable but she covered that up quite well.
"I think her problems showed themselves in school but I don't think that's where the problems originated."
Cry for help
Elaine's father Bernard has previously told the inquest that his daughter was a timid, shy girl, who suffered various forms of bullying.
These included name-calling, having her chair pushed from under her and having pencils stabbed in her back.
He believes Elaine's overdose was a cry for help rather than a deliberate suicide attempt.
Pathologist Mark Bennett said the cause of death was multi-organ failure, due to liver failure caused by the paracetamol overdose.
Elaine told medics she had taken 15 tablets, but the inquest has been told that the full amount would probably never be known.
The inquest continues.