The basic premise of your letter writers is that failure in school subjects should have a healthy (even ennobling!) effect on the child enabling him to face the reality of a future tough world out there. The basic philosophy of the writers is based upon a survival of the fittest doctrine that seems to be at work in the evolution of the species.
However, they forget two major facts about the world of nature: (1) the young are protected in a great variety of creative ways so that they can make it to adulthood, and (2) the natural environment presents an innumerable variety of avenues for the learning of successful coping.
In contrast, the school measures the abilities of children by a very narrow and limited range of their potential skills. Granted that they should all learn how to read, but nowhere in the psychology of child development is it written that they should all start reading at the same time or that they should all end up at the same place. The schools are engaged in a mass production effort, and the attempt to stamp out each individual child in a common mold and to measure the success of the product on some arbitrary ball-shaped curve (for every "A" student there must be an "F" student) goes contrary to the infinite variety that exists in Homo sapiens.
To label a child as an "F" student is the equivalent in our society as labeling him as an "F" person because the school is the only compulsory institution in the labeling business. For our society to establish a compulsory institution whose main function is to compel children to go through a selective process very early in their lives when their self-concepts are still being formed; and for this institution to determine that fully a third of them (see the dropout rates) will never make it as successful adults; and for this selective process to be based on a very inadequate and limited range of experiences is not only nonsensical and wasteful of human resources, it denies the sapiens in Homo.
The end result of an educational system based upon the natural growth and development of the child rather than upon some artificial ratings imposed by the adult world is that the child through his successes learns first how to respect his own abilities, and then how to cope with his limitations without succumbing to them. And of course the school's responsibility in fostering growth through success rather than failure is doubled when faced with today's increase in the number of children from those groups whose backgrounds make it extremely difficult for them to make quick and easy progress in an alien school system.