The incident took place on Tuesday night when the girl went to the char along with her ‘boyfriend’, said Inspector (Investigation) of Boalmari Police Station Md Shahidul Islam.
The ninth grader filed a case in connection with the incident on Wednesday under the Women and Children Repression Prevention Act.
According to the case statement, following a relationship developed over phone, the girl went to meet Nasir Mollah, 20, a resident of Babukhali village in Mohammadpur upazila, on Tuesday.
Nasir along with his four friends -- Tarok Mondal, 19, Shaikat Mondal, 18, Nasir, 30, and a tenth grader, 16, residents of Lankarchar village of Boalmari upazila, raped the girl in turns from 8pm to 11pm.
A team of police visited the spot on Wednesday afternoon.
The girl was sent to the One-Stop Crisis Centre in Faridpur Medical College Hospital.
Meanwhile, police arrested Tarok, Shaikat and Nasir (2).
However, the main culprit, Nasir (1), has been on the run, said the police official.