Each day during prolonged work periods such as center time, you should be able to briefly meet with each student to see how their work is progressing. If they keep all of their weekly work in a folder that they carry with them, it makes this meeting very easy. Bring a recording sheet to mark students' progress, and if they are progressing through their work satisfactorily, check their names off or draw a smiley face. :-) If they are not, then privileges such as free time or class parties, etc. have not been earned. You may want to enforce this daily or base it on their work for the whole week. These students need to finish their work while they rest of the class is doing the "fun" activity.