There's a good site that teachers hang out on: teachers.net or teachersnet.com. You'll see similar questions. We're all trying to figure out teaching motivation and guiding classes. I would only start with asking yourself if you are fast-paced; how snappy are you up there? If your speed exceeds theirs, or if you appear to them to be as fast as a speeding bullet, then they sometimes slow down a bit.
Another training technique I once learned was to look in the mirror each morning and form the most awful scowl imaginable. That is "the Look." Then you take the Look and hit them with it -- the meanest, nastiest possible ugly face. Sometimes that works. One successful teacher did not crack a smile from Sept. until May, and by that time she said she had them in the palm of her hand. I can't do that, but it is a good recommendation.
Usually, there are 1-3 wiseguys and if you can defuse them, the rest will be follow okay. I never wanted to believe that "The Lord of the Flies" stated truth, but classrooms can bring out the dynamics of mob behavior. Individually they are pipsqueaks, but together they are a united army. So perhaps go after the hardest nut to crack.
I speak only from experience, some of which could only be described as failure. Try to maintain your dignity irrespective of what asinine whirlwind is going on. Also, do a thorough job of marking up their papers in red ink and making suggestions. They like it when they see that you went over their work; they really do. Also, maybe use stickers. Even high schoolers like stickers.