The Class 10 student of G.D. Birla Centre for Education in south Kolkata's Ranikuthi was later declared brought dead at a city hospital.
Following the incident, senior officers of Kolkata Police and a team of forensic officials rushed to the school and collected evidence from the spot.
"The body of the girl was found from inside a washroom at around 3.30 pm. The door of the washroom was locked from inside. Some blood-stained blades, a pen and suicide note has been recovered from the spot. The plastic packet that was wrapped around the girl's face has also been recovered," a forensic official said.
Police said the body has been sent for an autopsy to determine the reason of the death.
"We have recovered a suicide note. The handwriting on the letter is being matched with the deceased. According to the letter, the girl was under mental pressure and could not sleep since the last three months," Joint Commissioner of Kolkata Police Muralidhar Sharma said.
"The message 'I Quit' was inscribed on the wall of the washroom from where she was found," he said.
The doctors at the hospital said there was a deep cut on the left wrist of the girl.
"There was no sign of life when the body was brought to the hospital. The left wrist of the girl was slit. It looked like a self-inflicted injury. More can be said once we have the autopsy report," a doctor said.
The girl, a topper in her class, reportedly went to the washroom at around 2.30 pm. The school teachers and staff started looking for her after she did not come back to the class after half an hour.
A classmate said the girl did not show any abnormality during her time in school.
"She is my classmate. I talked to her during the school hours today. We did not find anything abnormal in her behaviour. All of us are shocked," the girl added.
( With inputs from IANS )