Shocking FB photo shows a teenager being sexually assaulted

OK, so there's this photo shared on Facebook yesterday that's gone viral on the social networking site. It purportedly shows a bunch of teen boys sexually assaulting another boy. It was taken in Chicago and the alleged assault is seen in the rear of a selfie taken by a girl. Some say it's fake/a prank but others who are taking it seriously have contacted the Chicago police asking them to investigate the photo.