How about this reform of education: It is always possible to make some rough assesment to see if a student has enough background to learn a new subject, make it so dynamics that student can take certain subject only if s/he passes some test or assesment, no matter what age what grade s/he is. Then, I think both the teachers and students will be happier. If everybody is doing so, and make them understand each person develop differently, then I bet the self-esteem problem will not be issue instead could be largely improved. At least, students without sufficient background will not sit there first "feeling" bad for himself, then start to make trouble to ease his frustration and anger.
Each student does develop differently. I know there are researches in this area and would like to call for more detailed researches so that we can convince the public that, for example, there is no problem for kids 7th grade still doing addition and substraction (I know many still can't, but they are "forced" to learn algebra,...).
In my own experiences, I have seen students developed very slowly in the beginning no matter how hard everybody ( parents teachers himself )have tried, but later on suddenly open their mind and grow even faster then their same ages. The key point is, we were very careful not to have them face frustration toward math, and help them ease the frustration related to the peer pressure that every one in their age were quite ahead. As slow as they were, they were happy with their own math which still has constant success although the step may seems too slow from standard point of view. So, they have never really been afraid of math.
Of course this slow, fast process may not happen to every students, some are just slower. But the point is, would it be better for students to solidly learn what they are able to learn, rather than throwing them everything that they are not able to digest anyway, no matter how you do it because the background is not there. Otherwise, they will be so overwhelmed that they won't even learn what they are able to learn.