At any given time, you could be focused on a given sub-group, chosen not necessarily by ability (the girls at the back, the quiet boys, the ones who don't like the topic, the middle row). Members of this group receive the bulk of your spoken and written attention for a week or two, and then you move on. It's not so defined that anyone can notice it; it doesn't exclude others; but it forces you to spread yourself evenly, and it doesn't require you to try to be all things to everyone all the time. This group, which only exists in your mind, is questioned a little more than the others; its answers are responded to a little more than the others; it works with you on guided and shared work a little more than the others; it has its written work marked a little more thoroughly than the others; and then you move on.