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.