Joseph:
I apologize, I think my prior post is too harsh. What I mean to say is that the Gemarah & Rambam should at least create a Safek in your mind that you should not tell people they will burn if they embarass those who do not take care of their children, and perhaps it is even Muttar. The school should (IMHO) have an unbiased Rav whom they should ask, and exhaust all other options (and perhaps warn the parent), also based on the Rav.