Even if the insurance is not required (check with your school district - they may have a requirement) I could not dream of having a group without insurance. For example, I would not be on the board of a group that didn't have insurance to cover the directors. I would not open myself up to personal liability in case someone got hurt at school or some wacky parent (and it does happen) decides to sue me because they don't like the way we have handled the money. Think of all the ways your group would be interacting at school. Now think of someone who really wants to sue. Even if you didn't sponsor the event but just provided volunteers you could be sued.
Also, one-day event insurance is difficult to get. I just tried to do this for a fun run to benefit one of the kids at our school who is raising money for some special physical therapy. Our group couldn't sponsor (against our bylaws to help just one child) and the scouts wouldn't insure anyone but other scouts. I finally got a quote for $700 for one day. None of the local insurance companies would touch it. However, if we had been able to sponsor it our insurance company would have covered it no problem and at no additional cost.
As far as I know, PTO's are never required to have insurance. There is good information about the various types of insurance PTO's might want to consider on the "National PTO Network" part of this site.
