Have you read or heard about this in the mainstream media? Not me. I just found out thanks to a Spanish-language blog: Disidente del Capitalismo, which I happen to follow.
In the context of the growing student revolt that engulfs Latin America (largely driven by the attempt by the states ruled by the right to make education more costly and elitist), the police forces of Guerrero state attacked rioting students with live fire killing three students (two on the spot and a third one who died later at hospital) and brutally beating many many others, who surrendered to police terror.
Video in Spanish with images of the violence (from the side of the police):