The two girls are studying in standard IX and X of a local school.
According to police, the accused had got in touch with the girls on the phone and asked them to come out with them to a beach at Karunagapally in Kollam district.
While one of them was gangraped on December 4, another was raped the next day, police said.
The incident came to light after the girls returned to their class and informed one of their classmates who in turn told the school authorities, they said.
Some more persons have been taken into custody and are being questioned, police said.