There are some fantastic sites out there that can help you prepare your students for Write Path, I've listed some below. You could let them have a trial run too, using a paragraph from an earlier year.

http://oneword.com/ - great for a quick warm up, one word given and then you have 60 seconds to write about it , or use it in sentences
http://www.thestorystarter.com/jr.htm  - has a range of 2340 starter sentences to give ideas for stories

http://teacher.scholastic.com/activities/storystarters/storystarter1.htm - online story machine with seemingly endless options, in a fruit machine like site. Great fun for all ages!
http://creativewritingprompts.com/ - 346 varied writing prompts

http://5card.cogdogblog.com/play.php?suit=5card  - allows you to choose 5 flickr photos to build into your story