Ethos is a rhetorical device that appeals to the audience by showing credibility. Graff uses ethos here by relating his argument of street-smart intellectualism to his own adolescent experience.
Growing up in Chicago post–World War II, Graff was all too aware of the class struggle developing in his community. Though he personally had no interest in academic intellect, he was concerned about how being perceived as unintelligent might affect his future. On the flip side, to gain the approval of the working class "hoods," he was concerned about not seeming too intellectual. He needed to seem tough.
The irony is that Graff realized that through these conversations and thoughts about toughness and class difference, he was learning the rudimentary elements of intellectual life and rhetoric. This ethos argues his point that students should be allowed to mold their intellectual identities around subjects that interest them rather than those that have been standardized.