Law
I was former VPHR for an international call center company. the issue of what is profane actually had enormous impact on a great number of FCC regulations. As a result, i have spend countless hours defending, researching, and going to actual trials regarding the issue of profanity used (in this case in the workplace). All work was on east coast US and Canada.

With lots and lots of great legal minds, we were not able to defend our position as an employer why the person should be terminated, because we did not clearly, nor could we clearly outline was, and what was not, profanity.

i should think the argument you would consider making is not one of whether the word was profanity, but rather, if your client would have had a reasonable opportunity to NOT use profanity given the clearly vague rule at the school. If the school is not able to clearly define what is and is not, then how can the child be expected to know what is and is not a violation.

basically, this is how i got my MFing ass kicked all over the courts for firing someone for the use of profanity. :-)