I start with the assumption that kids are not alike but individuals with different learning rates and styles of learning. I was frustrated with school when I went and when I taught I had kids in a class that need remedial work and kids that could do much more. Unfortunately, I had only limited time and materials to deal with these problems. Outcomes will be different but optimal for each student.
My mother and aunt dealt with this problem in their first grade classes by having lots of materials and structuring their classrooms. There was only limited success and the problem increases as kids move on.
Districts could change school structure to help but such things as Common Core, NCLB, and a number of other programs that are tied to funding make that transition more difficult.  What is needed is the development of materials and methods funded by government that could be used by teachers and districts. I taught in a small district that makes development difficult.