I wonder when this could have happened without witnesses. I don't think it is a good idea to talk specifically about the problem to the class at this time. I think I would try to fit in a talk to the class about being able to talk to you about something that is bothering them. At third grade level, I would have to say this "pile" is not confusion about how to take care of yourself when you have to go to bathroom nor is it confusion about where to go. (Except on the outside chance that someone really had to go and the bathroom was in use and they felt they just couldn't wait) This probably was some sort of a "message." Talk to your school counselor, principal. Watch students carefully for any signs of anti-social behavior. Is there someone who is extremely anything.... shy, violent, friendless, new, different, neglected....Do you have any suspects?