Teaching students what to say when they don't know what to say.  John Seidlitz and Bill Perryman has 7 easy steps in their book 7 Steps to a Language Rich Interactive Classroom. My favorite is their poster that can be hung in your classroom. It provides some easy go to steps to help students that say "I don't know" in your classroom. In my classroom "I don't know" is not an answer and I always point to the poster. Students can choose any one of the helpers from the poster. 

Check them out on the website http://www.seidlitzeducation.com/7-steps-to-a-language-rich-interactive-classroom