She left the school because she felt ostracised by her schoolmates.
Khoo Karmun, 17, also saw a psychologist and was diagnosed with depression.
She had attended Raffles Girls' School (RGS) from 2011 to April 2013, when she left to study in Australia. She is now in her final year of high school at Carey Baptist Grammar School in Melbourne.
More students, especially girls, report that they have suffered from verbal and relational bullying, said Ms Rachel Tan, one of the centre directors of the Singapore Children's Society.
Relational bullying involves covert behaviour that hurts others emotionally or socially, such as making fun of or gossiping about others, or excluding a person from a group, counsellors said.
Karmun said she was called to a disciplinary hearing in 2012 before two teachers and two Secondary 4 prefects. She was a trainee Secondary 2 prefect, on her way to being a full-fledged prefect the next year.
"I was told that I was too friendly with my juniors and I wasn't professional enough. For instance, they said that laughing with juniors was inappropriate," she said.
"Another trainee prefect in my class also told them that I had been eating in class - which wasn't true - so my pencil case was checked."
She eventually decided to leave the prefectorial board.
Another incident happened when she made a switch in her co-curricular activity, from the handbells ensemble to the drama club, at the end of Secondary 2.
She was accepted into the drama club in September 2012 but, soon after, received an e-mail from seniors saying she had been rejected.
She clarified the matter with the teacher-in-charge, who told her she had been accepted. But she said that many members did not talk to her after she joined.
"Sec 1s and 2s are usually stagehands who carry props during rehearsals, while the upper secondary students are actors and directors," she said.
She felt they were probably unhappy that she had bypassed the hard work by joining later.
"My self-esteem and social skills suffered," she said. "I didn't want to take part in class anymore, and I became more rebellious and got scolded by teachers a lot more."
She had nightmares, could not sleep well at night and had episodes of blackouts during the day.
In March 2013, she was diagnosed with situational depression and went on medication for six to eight months and therapy with a psychologist before she recovered.
"Part of the stress in school came from the culture of wanting to be the best at everything," said Karmun, a straight-A student who was in the Gifted Education Programme.
Her father, Mr Gary Khoo, 45, said he and his wife noticed that she went through "mood swings".
"She would cry at home, not talk to us and was very emotional. But we thought it was just her teenage years, so we were not too concerned," said the director of education services at an IT firm.
"But we realised how bad it was when we discovered farewell notes in her bedroom drawer. It was heartbreaking and we took her to see a counsellor," he added.
"She's outspoken and fairly critical in her thinking and is unafraid to correct people when they're wrong. Unfortunately, if you're different, you get picked on," he said.
The parents tried to transfer Karmun to another local school, but were told by several schools they could not take her in at Secondary 3 to 4. They applied to Stamford American School here but were told by the Ministry of Education that the application was unsuccessful as she was a local student.
A spokesman said students who wish to transfer to other secondary schools will be assessed on a case-by-case basis. Similarly, Singaporeans who apply to international schools are allowed to enter only in "exceptional circumstances".
"So I made the hardest decision, to split the family up and send her abroad," said Mr Khoo, who works here while his wife and daughter are in Melbourne.
An RGS spokesman said it was aware of Karmun's case, but could not "share more due to confidentiality of information".
"RGS and its teachers have always acted in the best interests of its students," she said.
When asked, former schoolmates of Karmun said they did not want to comment on the past incidents.
In April this year, former RGS student Cheryl Tan, now 18, sued the school, claiming it failed to protect her from being bullied while she was in its Chinese orchestra in 2012. She also said this forced her to study overseas - at the Wells Cathedral School in England, where she is completing her A levels.
RGS has denied the claims, saying that it found no bullying after investigating her complaints.