Keep enough chaos in your classroom so that the slow kids don’t know they’re slow and the fast students don’t know they’re fast. One easy way to do this is through multiple puzzle-sheets and avoiding a linear progression through them. Let’s say you have puzzle sheets A (starter) through E (difficult). After the first students (the fast ones) are completed the starter sheet A, they immediately go to D. The next group of students go to C after completing starter sheet A. Remember that slow students are sometimes as capable or more capable of solving hard problems.