If you see them cheating during the SAC and it's something the teacher could catch them red-handed for (e.g. hidden notes), then call it for sure. The rest of the non-cheating class would be glad you did it (If the whole class is cheating though... then perhaps you oughta stay quiet and report it later, unless you wanna get beat up. Or better yet, cheat as well). And while the cheater would be pissed at you, it's only because they got owned by the hammer of justice. If the SAC is over and you have no proof though, then don't do it.
It's as much the teacher's fault for letting the student cheat as it is the student's for cheating. If your teacher is sleeping on the job then report the teacher to someone higher up. But whatever you do, don't let your teacher know you did it.