I can't digg up the original contract to check; but some of the stories state that they are going to Apple because the deal was to purchase 'iPad+software', as a packaged product, from Apple. By all accounts Pearson was the significant weak link (not a shock, that's pretty typical for them), while Apple's stuff suffered only from the fairly pitiful state of iOS management; but the school district didn't structure the deal as 'Contract #1, buy ipads, Contract #2, buy textbook apps'; it was a package, and their claim is that half the package was rotten and the other half is of little use to them without the underdelivered component.
Given that Apple is reputed to be a brutal and efficient taskmaster of its suppliers, I'd imagine that either the school district will fail, or Apple will gouge it out of Pearson; but to the best of my understanding there is logic behind complaining to Apple, given the terms under which the devices were purchased.