I read Dumbing Us Down, which might not be the book you're thinking of -- if memory serves, he's written a few -- but I don't remember him ever quite going with the conspiracy theory angle.
He skirts the edges of it pretty closely, granted. In the context of history of education, his thesis is basically that the American educational system is an offshoot of the Prussian system and that that system was picked because it prioritizes obedience to authority. Even if we take that all at face value, though, it doesn't require a conspiracy -- just a bunch of 19th- and early 20th-century social reformers with a fondness for one of the more authoritarian regimes of the day, openly doing their jobs.
Now, while it's pretty well documented that Horace Mann and some of his intellectual heirs had the Prussian system in mind, I've never seen historical documentation giving exactly those reasons for choosing it. And in any case the systems diverged in the mid-1800s and we'd need to account for subsequent changes before stringing up the present-day American school system on those charges. But at its core it's a pretty plausible hypothesis -- many of the features that after two World Wars make the Prussians look kind of questionable to us were, at the time, being held up as models of national organization, and a lot of that did have to do with regimentation of various kinds.
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