The student at Cleveland Middle School called the teacher's attention to the lurid video of the incident that occurred late last year, The New York Times reported Wednesday.
Police said the video led them to the abandoned trailer home where the rapes took place and eventually to the arrest of suspects.
In the past month police rounded up five Cleveland High School students, including two members of the basketball team as well as the 21-year-old son of a school board member.
Others suspected of involvement in the rape have criminal records for selling drugs, robbery and, in one case, manslaughter. A television station earlier reported as many as 28 men and youths may have been involved. Eighteen have been charged so far.
Suspects range from middle school students to a 27-year-old.
Police have declined to discuss the investigation.
A spokesperson for Cleveland Middle School said the district's security department interviewed the victim, determined that a rape had taken place and turned the matter over to police.