Personally, I really dislike the notion that "you have to have a question ready to ask." I'm a very analytical type who researches things compulsively ahead of time, so I honestly didn't have something that I *had* to ask. However, you gotta play the game...
Definitely stay away from questions that have obvious answers that you could have looked up on the school's website.
I chose questions of the "what's your opinion" variety. For example, Kellogg has a reputation for being teamwork zealots. I asked "I have a couple friends who went to Kellogg. One said the teamwork environment was great. The other said that it can sometimes be a bit extreme and forced. What do you think? Is this changing?"
So... questions that are very much about the school, demonstrate that you're thinking about the school, and don't have obvious and available answers.
My 2 cents