John/ Norman,
First may I say I understand fully that the Wikipedia for Schools is nothing like as good as a resource written for the classroom by teachers, like Spartacus, and never could be. We do offer a few lesson plans around geographical topics (they are downloadable here) which are written by teachers who kindly offer their time to us as volunteers. But the prospect of writing 20 volumes like that is another issue altogether.
Second, on the selection, the Schools Wikipedia was put together as a collection substantially by volunteers (a few retired teachers, some people working for UCLES which is nearby and some students at the university) . They were given a process to pick the best available version of an article on the English Wikipedia and then edit it by deletion only. We did not get into the much harder task of creating and rewriting content. Some of it like the Somme has a reading age of 16+. However there are some "elementary articles" and at our request Wikipedia themselves created some portal pages on topics coinciding with curriculum topics such as Early Modern Britain and so on. These pages now exist on the main wikipedia site too. We are planning on a "tour bus", running through a series of articles on a topic. Much of the material though is more fodder for project work for older children.
All in all though I take comfort from John's comments: if this is free and can be put on a school intranet for children why not? If it just helps a percentage of children who are more self-educated that's already a win and we don't aspire to more than being helpful.
Andrew