Her mum, Elspeth, a GP, said her daughter "appeared to have a real fear of puberty and the fear of growing up".
The teenager, who attended £5,910 per term King's College in Taunton, Somerset, as a day pupil, was found hanging on July 14 last year at home, by her father, John, a pathologist.
Dr Geraghty told the inquest that Fiona had difficulty in settling in at school after starting there in September 2010.
"In the first term Fiona did have some relationship issues with girls in her peer group," she said.
"Fiona had a very poor body image."
She told the hearing she was first alerted to the possibility her daughter was suffering from bulimia when she was contacted by Fiona's housemistress in February last year.
"She said she had seen Fiona vomiting," she told the hearing.
Dr Geraghty said she was concerned because there was a history of eating disorders in their family and took Fiona to see their GP.
"Fiona said she started vomiting following taunts about her size," Dr Geraghty said.
The GP referred Fiona to the Child and Adolescent Mental Health Services (CAMHS) in Taunton, where she was seen four times before being discharged.
Dr Geraghty said her daughter, who was achieving a string of A and B grades at school, had a couple of minor disagreements with her parents over messages she posted on Facebook in the weeks before her death.
The court heard the cause of death was hanging.