There are SOOOO many kinds of questions that can be great for a circle, it all depends on your goals and your group. Is it a quick check-in circle or a longer sharing circle? Do you have 5 people or 25? Are circles new to them or has this group been doing circles together for months? Are they 5th graders or 11th graders? Do you have a specific topic or issue that needs addressing? (If the purpose is to address a specific conflict, please contact Molly or Jill.) .
A great way to get students involved and invested in the circle process is to invite them to submit questions. Maybe have a box where they can drop questions any time. You'll want to give these a quick review to remove anything really inappropriate, but remember, silly can be great for building community. After some experience you can invite students to take a turn as Circle Keeper as well!