How many of us have let kids just check out with an "I Don't Know," and then we move on to someone else? We cannot do this!
In my class, students know they cannot get off the hook with IDK. They have options. They can Phone a Friend, where they get to have someone else give an answer, then they must confer with that friend and decide if they want to take that answer or if it sparked some other answer. They can Tag Out, where they get to call on a friend to answer the question completely knowing that the next question asked is coming straight back to them and then lost this option for the rest of the class. They can Buy More Time, where they can choose for you to move on to another question or whatever in class and they have to get back to you you (in 5 minutes, by the end of class, ...) with an answer. If they choose this option, it better be the right answer since they had more time to think or check resources. Their final option is to get a Classmate Clue, where a classmate can give them a clue, not the answer, and then they will be able to answer.
These have to be taught to the students when you first start classroom discussions. That way, they options come naturally to them later and everyone is one the same page when we are all talking. I made a poster set for my classroom to remind the student of their options once we have learned them all.
Click on either picture to get the posters for your class. I hope you find it helpful for getting those "I Don't Know"s out of your classroom!