This is great and it fulfulls two important criteria: free and no need to register. We don't have a photo editing package in school so this is brilliant as we have been using the DigiBlue software which is no where near as good as this for photo editing.
They have demo photos you can use to have a play with the tools or you can upload your own. The photo can then be saved to your computer, rather than on the site (however this is an option if you register). It also has option to create "scrapbook" style pages - which look great - and slideshows (although Photostory is much better for this).
It does all the things you would expect from a photo editing package, as well as having lots fun features like adding writing or pictures on top and adding borders and frames. There are also "premium" features which you would have to pay for but I don't think for school purposes that would be necessary - there is more than enough there for free.