A PM to another teacher reminded me of what I do when half the class is completely lost and the other half understands what I'm doing: I come in with a fun, self-explanatory assignment for the kids that "get it." It's to be done in class, quietly, and finished as homework. (They NEVER get to do homework in my class, so that's incentive enough to be good.) At the start of class I explain what we're doing. Those that "get it" will have an assignment and move to the back of the class to start it. For the rest of the class, it's a massive extra help session in the front. They can sit wherever, and I go through the material in the tiniest increments I can, stopping everytime someone has a question or doesn't know what to ask. We may only get through 2 or 3 questions, but it normallly helps tremendously. I seldom need one of these days; many years I don't use them at all. But they're a tremendous resource to have in the back of your mind, particulalry in math. Everyone leaves happy. The kids in the back have done their homework. The kids in the front are much more attuned to the material and able to do the homework.