Reach out to the school. More than likely, they have that rule in place because they can't have a whole classroom of kids who aren't potty trained—otherwise, they'll spend a lot of time changing diapers. But if your child is the only one (or one of a few) who isn't potty trained yet, they may be able to work with you on getting her up to speed. In fact, exposure to other kids using the potty can be just the thing she needs to finally take to it.