The plight of the girl, a sixth standard student, came to light two days ago after she revealed her trauma to child line counsellors who had visited her school.
Medical examination of the minor at the Manjeri medical college has revealed that she was sexually abused.
Police have prepared an FIR in which 17 persons are named as accused, of which 12 were arrested today.
The girl has been sent to a juvenile home at Kozhikode.
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Police have registered case under Protection of Children from Sexual offences Act and Juvenile Justice Act.
The girl's stepfather and mother used to take her to different places for prostitution, she reportedly told child line volunteers.
Her stepbrother has also been detained after police received information that he had also allegedly abused her.