It turned out that the business model for educational content creation (in “language arts,” at least) is rather less romantic than I had envisioned. Content assignments are farmed out to an army of small-time contractors with some kind of nominal qualifications — stay-at-home moms, former school librarians, and so on. Overwhelmingly, the assignments are not story ideas or anything the like; the basis is grade level, mean word length, schematic adherence to state educational standards (see above), and volume, volume, volume. I think the pay is per passage, rather than by the word.
The result is a stream of unmitigated crap, crap that is unremitting and remarkably homogeneous. It took a small army of editors (myself included) to cull them into readable shape, although “polish” and “readability” didn’t seem to be the first priority in the division where I worked. (A major reason why I wasn’t kept past my initial probation period was that I slowed the process down by making too many requests for edits.)