My father has worked in a fair few educational insitutions during his career as a primary school teacher.
And in each one, the phenomenon of the 'phantom shitter' has always presented itself. Its a queer thing, but apparently if a kid (it's mostly boys, if that shocks anyone) is having some kind of problem at home, or with social interaction, or whatever, they often take to laying pipes in inapropriate places, or smearing it everywhere, or both.
So, it's not uncommon, and should signal a strong warning sign to teachers that the child has emotional problems. To put it another way, some kid is utterly fucked-up, and seriously needs his twisted face rubbing in it, the filthy shit-easy freak.