As many as 12 persons were taken into custody with regard to the shocking incident. While four of them were arrested yesterday morning, four others were nabbed later in the night, police said.
Three women, including the step mother of one of the victims, are also under custody and being questioned, they said.
According to police, the accused got in touch with the girls, studying in ninth and tenth standard, on the phone and asked them to accompany them to a beach at Karunagapally in Kollam district.
The incident came to light after the girls returned to their school and informed one of their classmates, who in turn told the school authorities, they said.
Police have registered a case under the Protection of Children from Sexual Offences Act (POCSO) in the incident.