"It sounds unlikely that any abuse could take place there." I have to disagree about the nuns...you've heard about priests, no doubt? Nuns don't have any special traits, despite what religion wants us to believe. Not to mention there are likely priests and maintenance men at the school.
The story about a non-existent child touching her is VERY suspicious. Even more alarming is her attempt to get YOU to touch her there, as if it's happened before.
If you really care about your child, you need to have her checked by doctor to be SURE nothing has happened; her actions are WAY too indicative of sexual abuse to minimize them. Good luck.