Not only was the new form of institution spiritually dangerous as a matter of course, but school became a physically dangerous place as well.
What better way to habituate kids to abandoning trust in their peers (and themselves) than to create an atmosphere of constant low-level stress and danger, relief from which is only available by appeal to authority? And many times not even then!
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Horace Mann had sold forced schooling to industrialists of the mid-nineteenth century as the best "police" to create moral children, but ironically, as it turned out in the twentieth century, big business and big government were best served by making schoolrooms antechambers to Hell.