Paragraph 17: What is wrong with the flesh of schoolboys?

From the text:

But with due deference to so excellent a friend and so deserving a patriot, I cannot be altogether in his sentiments; for as to the males, my American acquainteance assured me from frequent experience that their flesh was generally tough and lean, like that of our school-boys, by continiual exercise, and their taste disagreeable; and to fatten them would not answer the charge.