It is beyond the capacity of any one teacher to teach a whole class of students each at his or her own learning edge. Much important information would remain untaught if there were no standard curriculum grade by grade. Many students benefit from learning in the company of other students - together they hold discussions, plan and present programs, etc. However, there is still room for individuals' needs and interests. Advanced students are given extra projects and assignments, sometimes as a group, while slower students are given make-up assignments or are put on a separate track with their own workbook.
Help is enlisted from home.
No one has learned how to prevent slower children from comparing themselves unfavorably with advanced children. However, teachers don't have to reinforce these unfavorable comparisons by a harsh grading system. Grade for student effort and application. Broaden the curriculum to include special projects, and include the results in reports home. Some parents, recognizing the enormous individual differences among students and seeing the harm done by unfavorable comparisons, have chosen to educate their children at home, where work can be given at students' learning edge.