Sub-Divisional Police Officer Kanteswar Gogoi said the girl from Nalbari came to meet the boy, who had befriended her in past five months over phone, at Ramdiya Bangalpara under Hajo police station and watched a theatre.
The boy, who introduced himself as Rahul Das, was actually Fakhruddin Ali, who had offered to drop the girl home but instead took her to a school building, Gogoi said.
He and seven of his friends, who had already assembled there, then allegedly took turns in raping the hapless woman, the SDPO said.
The rest five, including Fakhruddin, however, managed to escape.
The trio have been identified as Alam Ali, Jaimuddin Ahmed and Hashen Ali.
Preliminary medical examination of the girl confirmed the crime. Detailed tests will be conducted tomorrow at Gauhati Medical College and Hospital, Gogoi added.