Some thoughts: Ask yes or no questions, or questions that have a finite answer set. Learn to ask the questions in a way that your vocal intonation doesn't give away the answer immediately. Call on a specific child and ask him/her to "take a guess" at the answer to the question if it's too hard for them to know (thinking about questions can be really useful even if you aren't going to find the answer), or say "here's a challenge for you: can you figure out...", or "given that X, what do you think Y is going to be". Find a system that spreads the embarrassment of answering questions evenly over the class. Emphasize this fact if it will mean children get made fun of less ("I'm going to ask you as many questions as it takes to get you to screw up too!", "If you laugh at someone else's answer, then you're next.")

The true/false or multiple-choice model could also help with the written prompt issue.

Also, this is pretty cool: http://web.archive.org/web/20090321202807/http://www.garlikov.com/Soc_Meth.html