At the very beginning of "Theme for English B," we hear that our speaker's assignment is supposed to result in some sort of truth. Writing a page from his heart will result in a page that, his instructor posits, will be "true." Nifty, huh? Well, our speaker doesn't know if he agrees with that, and thinks that truth is a lot more complicated than something you can just sit down and churn out. However, as he thinks things through in the poem, we'd wager to say that he definitely hits on some fundamental truths: about race, about America, about identity. What comes out of him, then, seems to be exactly what the instructor asks for in the assignment. True that.
The speaker feels that it's difficult to find truth in a country he finds hypocritical. It's like searching for a truth needle in a haystack of lies.
Though the speaker initially doubts that it's easy to find "truth," by the end of the poem he has had some truthful insights. In doing so, he's makes his initial self out to be a liar.