The "new" version of Google Sites (it has been out for two years now) has seen a steady increase in features over the last six months. Many of those features are things that existed in the old version of Google Sites and are now available in the new version. The latest feature it make it over from the old Google Sites to the new version is the option to duplicate a site.
Duplicating a Google Site will let you make an exact copy of an existing site and have it reside at a new URL. To do this just open the "more" menu (the little menu just to the left of the publish button) and click "duplicate." You can then change the name of the duplicated site. Your duplicated site won't be live on the web until you click the publish button on it.
Applications for Education
Duplicating a site could be a convenient option to use at the beginning of a school year. If you spent the previous school year maintaining a site and you're happy with the look and content, you could re-use it by duplicating it and then just updating parts of it through the year.
Duplicate sites could also be helpful for testing a new design or new feature without impacting your primary site. In other words, you can use the duplicate as a test site.