I've spent some time creating sites and pages within Google Sites, and I think its potential for schools is huge. It's a shame that it only functions as part of Google Apps instead of a Google Account, but once you get past this, you can really start to create.
Take, for instance, the ability to insert any Google Doc, Spreadsheet, Presentation or Google Form directly on to a page:
Google Sites seems to have most of the same functions as other wikis, although you need to spend a little bit of time getting used to the different layouts if you're used to wikispaces or pbwiki.
Google Sites has the potential to be the main portal for any school, with new sites created within the one central site.