Children sometimes just make things up. Often there doesn't seem to be any motivation (that an adult would understand), they just do it. I assume the school is familiar with this. BUT - with the world so paranoid about child abuse it is probably a good idea to contact the teacher. As someone has mentioned they may already have started an investigation.

I don't want to be too alarming, but this sort of thing can become very serious. My wife an I were foster parents for a brief time. The 5 year old girl with us had been taken out of her house and kept from her parents for > 6 months based on something she said. We were convinced that nothing at all had happened, but once the process was started there seemed to be no end. We did everything we could to try to get that poor girl back with her family - to no avail. As long as there was "any possiblity" of a problem they kept her out of her house.