"And this will be exactly what he tells me about the assignment. That she didn't ask him to make it a certain number of words, that it had to be positive, etc. If it wasn't defined, he assumes he can approach it as he wishes."

That is awesome! Some people spend hundreds of thousands of dollars and years in law school to learn to think that way. He is doing it at twelve. Bravo!

I worry about having a kid in school because I look at these situations like your son which is obviously not acceptable to the teacher. I would have shot back the following response:

Thank you for bringing my son's response to your assignment to my attention. I am assuming you are taking issue with his use of the words "hate" and "sucks" as well as the negative tone of his response. I will discuss these issues with him. However, to avoid similar situations in the future, perhaps you could send me a list of words you find objectionable. Unfortunately, my son has been exposed to some colorful language in his lifetime. I would hate for this to happen again.
As for the issue of tone, I think more detailed instructions would prevent misunderstandings in the future. I think something like this might have gotten you the results you were after: Please write about your favorite myth using at least 250 words and in a manner that makes me feel good about my curriculum choices.
Thanks again for bringing this to my attention.

Anyway, apparently my junior high past time is alive and well. smile