Back talking sixth graders?

I'm entering my second week student teaching in a sixth grade cl and still don't know what to do about the kids' back talking. Clroom management is definitely not one of my strong areas, but even when I try to be firm and ask the students not to do something, they will turn it around and say, "No, I'm not doing that." or just shrug it off. Punishments such as keeping them in from recess don't seem to work. I'm going to be teaching more subjects soon, and I'm really worried about the outcome. Any advice?