Hi there

Firstly an apology if this has been addressed previously, it is proving very difficult to search for K2 questions as the search disregards it is too short.

I've recently installed the paid version of JoomSEF 3.5.5 and added the plugin for K2, which all my content is in.

When I set the K2 settings to Add category to URL = All categories, there is an additional category appearing in the URL.
For example, mysite.com/school-news/news/school-news/name-of-item
where it should be mysite.com/news/school-news/name-of-item

If I set Add category to URL to Only last one, I get
mysite.com/school-news/school-news/name-of-item

And if I set Add category to URL to No, I get
mysite.com/school-news/name-of-item, which doesn't signify which overall category the page is from.

I am purging the URLs in between each change. Additionally, when set to the first two options, it is breaking the CSS on the pages (is fine on the last option for some reason).

Any help?

You are probably adding menu alias to the URL, you need to configure it so it doesn't. Go to Manage components, then select K2and select the Common tab. In there, set Show Menu Title to no. For the CSS, it might be that it can't find it. Check to see if the CSS is correctly loaded by using Firebug or similar. It might help changing your base href settings (I use No - leave original so that anchors are properly handled).