Baker surely knows this. “It was easy for me to be ‘cool’ by making a few mildly subversive references,” he says at one point, “but they” — teachers — “had to keep a lid on the lunacy day after day.” He could have gone further. His fleeting encounters with “a thousand kids” do not enable him to see any of them grow, or fail to grow. He writes no lesson plans, takes home no papers to correct. Unlike virtually every adult he encounters, he does not (we assume) need the job. Nor is he ever in much danger of losing it. The ­classes in his charge frequently degenerate into lesser versions of a cafeteria’s “full ­riot-gear fluffernutter death-metal maelstrom,” but he is called back to sub again and again. As for his “mildly subversive references” — consisting mostly of jabs at the value of what he has been asked to teach — they affect his paycheck to about the same degree as belittling John McCain’s war record affected Donald Trump.

The lives of teachers are not so much Baker’s concern, however, as the burdens of backpack-laden kids. The constant barking of the P.A. system, the successful completion of work sheets as the seeming “aim of life,” children overwhelmed by tasks they have no idea how to do, children ­overmedicated to the point of hallucination (one on so much Paxil that he’s hearing voices in his head), a class of third graders expected to address their routine problems (“I have to go to the bathroom, but somebody is already out and it’s an emergency”) by using a scanner app on their almighty iPads — Baker puts it all in our face. He also includes some delightful encounters — with a charming boy who loves riddles, for example — but they only add poignancy to a question Baker poses to one of his ­students: “Is it an engine of oppression, school?”

He leaves the answer to us. For such a long book, Baker makes few recommendations, most of them modest enough — shorter days, less homework, more attention to foundational knowledge (including how to spell). As a rule he’s on firmer ground when he questions sequence than when he balks at scope. His remarks on the folly of “prematurely forcing kindergarten kids to write” would get a thumbs up from anyone with a 5-year-old or a heart. On the other hand, when he suggests to a science class that terms of taxonomy are deliberately obscure and probably useless, the reader wonders whether he’s joking. He isn’t. On his last page and out of earshot of any students, he tells us: “There are no key terms. There are no themes, no thesis sentences. There are no main ideas.” This would make a fine epigraph for a postmodernist novel; I’m not sure it makes the best credo for equipping a young person to read it.

Baker rightly perceives the detrimental hands of “educational theorists” and corporate opportunists (a grossly biased resource on nuclear power is included in one of his teaching packets) in creating schools where “mind and soul” are reduced to “dead meat.” What he may not perceive is how the programmed inanities of the School Reform movement are partly an overreaction to the very sort of teacher Baker at times typifies to the point of burlesque: the blithe boomer oracle who just wants children to “be themselves,” who hates “separating people” even if they’re making it impossible for other people to think, who stands in the midst of pandemonium munching an apple while Donovan’s “Sunshine Superman” plays in his head. “Did you used to be a hippie?” a student asks Baker. I beat that kid to the punch by about a hundred pages.

But any deficiencies one might find in Baker’s classroom management, and he finds plenty on his own, are not all his fault. “Substitute” reminds us how even the best teachers can only be a little better than the schools in which they teach. “You survived,” an assistant principal says to Baker at the end of a trying day, an outcome any administrator worth his share of oxygen would have guaranteed by showing up in Baker’s classroom no later than 8:30 that morning. Never happens. Nor does it occur to anyone that a writer as accomplished as Nicholson Baker might be put to better uses than a review of the template for constructing a “hamburger” essay. Why not invite Stephen Hawking to a physics class so he can recalibrate the scales?