Most of us make a point of putting our school days out of mind as soon as they’re finished, so it can be hard to remember just how difficult the teacher-student relationship really is. High school students take their teachers for granted, treating them as a part of the school’s machinery, like the cafeteria and the hall pass. That is why there’s nothing more embarrassing, even uncanny, than running into your teacher outside of school – it seems like a category mistake to find a homeroom teacher on a date at the movies.

But while this indifference is partly due to simple teenage egoism, it is also a necessary defensive technique. Teachers wield excessive influence over a student’s life – not just the ability to give or withhold good grades, but the privilege of setting the emotional climate for every hour the student spends in class. A bored, resentful, vindictive teacher can poison a child’s life. Whether it is a cause or a result of the much-lamented low status of teachers in our society, there can be no doubt that a significant number of teachers are poisonous in this way: It is hard to get through 12 grades without encountering at least one.

What is less easy to recognize is that a good teacher, too, can be a kind of antagonist. Or, rather, not a good teacher – not the kind who is an expert on his subject and knows how to share that expertise – but a “good teacher,”the kind who approaches the job as a starring role. In the movies, this sort of teacher is always the one who breaks through the students’ stony facades and manages to leave an imprint on their souls – like Robin Williams in “Dead Poets Society.” In reality, however, such a teacher’s performance is a kind of emotional manipulation, a suffocating demand for affection and attention.

The narcissism of such a teacher bursts the reasonable bounds of the teacher-student transaction, which is designed to afford both sides the refuge of their indifference. Ostentatious charisma, in a teacher, is nearly always a danger sign: The teacher who needs to be loved by hundreds of teenage strangers has an unstable ego, which is liable to manifest itself in less charming ways. On the evidence of “Teacher Man” (Scribner, 258 pages, $26), the third installment of Frank McCourt’s memoirs, this is the sort of teacher Mr. McCourt was.

His 30 years of experience in the New York City public school system is a central part of the McCourt legend, but it is still hard to reconcile the anonymous English teacher with the celebrity he has become. In 1994, Mr. McCourt published “Angela’s Ashes” – expecting, as he writes in the new book, that “it might sell a few hundred copies and I might be invited to have discussions with book clubs.”

Instead, it became one of those unaccountable smashes the book business dreams about – millions of copies sold, Pulitzer Prize, Hollywood adaptation, the works. The story of Mr. McCourt’s miserable, impoverished childhood in Ireland was followed by “‘Tis,” which carried him through his first years in America as a bewildered immigrant, menial laborer, soldier, and student. Now, in “Teacher Man,” Mr. McCourt mines his decades in the classroom for another lode of stories.

What makes “Teacher Man”fascinating is the subterranean conflict between Mr. McCourt’s carefully staged persona and his actual personality, which keeps breaking out in spite of his best efforts.The character Mr.McCourt, honed over two previous books, is designed to extort our approval. He is put upon, hapless, lonely, sad, yet always charming and resilient, always needing and deserving love. In short, he is the perfect image of self-pity, and his popularity shows how universal and voracious our self-pity really is: Even if you never starved on the streets of Limerick, you can’t help seeing something of your victimized self in Frank McCourt.

The ease with which this character translates into the classroom suggests that it was in the classroom that it first developed. Mr. McCourt’s particular combination of theatricality, neediness, and self-righteousness flourishes in front of a group of teenagers, who could hardly resist an appeal to these constituent elements of the adolescent self. From his very first day in the classroom, Mr. McCourt tells us, he was being mistreated by both his students and his superiors.

It is March 1958, and the 28-year-old Mr. McCourt is facing an 11th-grade class at McKee Vocational and Technical High School on Staten Island. One rowdy student throws his bologna sandwich at a friend, and Mr. McCourt, unsure how to restore order, hits on the idea of picking up the sandwich and eating it. For this, he is reprimanded by the principal, but the odd impulsiveness of the act wins over his students.

This first anecdote contains the seeds of the teacher Mr. McCourt becomes over the course of “Teacher Man.” He impresses himself on a bunch of recalcitrant students by his sheer creative boldness; then he has to defend this quicksilver charm against the plodding, rule-bound bureaucrats who lie in wait for him. Even if this description of things were basically true, however, Mr. McCourt exaggerates it into a melodrama that can’t help but be false. All the people we meet in “Teacher Man” sound like they came out of a sitcom, and eagerly caricature themselves.

The bologna-scolding principal riffs on his own stuffiness: “Think of the problems we’d have if teachers just dropped everything and began to eat their lunches in class. … If we’re not vigilant these kids, and some teachers, your colleagues, young man, will turn the school into one big cafeteria.”

The students riff on their apathy: “Hey, everybody, we gonna study the paragraph, the structure, topic sentence an’ all. Can’t wait to tell my mom tonight. She’s always asking how was school today. Paragraphs, Mom. Teacher has a thing about paragraphs.”

And Mr. McCourt riffs on his naughtiness, as when he instructs a class to spend the hour reading recipes out of cookbooks: “David, I want you and the class to note the time and date and the fact that in room 205 of Stuyvesant High School you recited to your peers the first recipe of your life. Only God knows where this will lead you. I want all of you to remember that this is probably the first time in history a class in creative writing or English sat together and read cookbook recipes.”

The recipe-reading is one of the anti-utilitarian spiels by which Mr. McCourt signals, to his students, and the reader, that he is No Ordinary Teacher. But in the nature of things, such disruptions to routine can only be exceptions, and “Teacher Man” gives oddly little sense of what it is like to teach the banal subjects, like grammar and vocabulary, day in and day out. Even in so short a book, Mr. McCourt’s actual classroom experiences are bulked out with stories about his childhood, his marriage, and his doctoral studies in Ireland. It is as though, having triumphantly escaped from a job that never really satisfied him in the first place, Mr. McCourt hesitates to return to it even in imagination.

“You’re getting older,”he reprimands himself late in the book,”and aren’t you a two-faced blathering mick, prodding and encouraging kids to write when you know your own writer dream is dying.” Who knows how many long-serving teachers greet each day’s class with that same blend of resentment and despair? Mr. McCourt, the book suggests, was the rare dreamer whose dream came true: He got to stop being a teacher.