This is a complicated passage and not one I would recommend for language learners. I applaud your efforts. "By the time one has started humiliating the student, the lesson is over. Nagging breeds its evil twin, Shirking.The other pretends to read the paper, goes upstairs and feels righteous." In this passage, the author personifies "Nagging" and its evil twin, "Shirking." The "other" refers to Shirking. To understand the sentence, you must know the meaning of "shirk." http://learnersdictionary.com/definition/shirk What the author means is that when a teacher nags or humiliates a student, the consequence is a student that shirks his/her responsibility. So the shirker/student feels self-righteous when he/she goes upstairs and reads the newspaper instead of doing his/her school work.

Its not quite clear who "the other" is. Maybe with more context it might make sense. It seems like "the other" might be referring to either the student or the one bullying the student, but its not clear.