In reply to:

that is the argument ad ignorantium.

It's evidence that's the thing, yes?

Without evidence either proving or disproving, both sides of the arguments would be arguments ad ignorantium.

And this is such an interesting situation to consider. I agree with Bingley that these kinds of arguments are made too often. And, Faldage, I'm reminded of my own incorrect proposition about down's being an interjection--and you said I couldn't say that it was so simply because I said it was so. Mine was an instance, wasn't it (without formally arguing) of an argument ad ignorantium? Or at least a proposition ad ignorantium. However, I think propositions might be casually taken to be arguments, yes?

But your point: You can't say something is simply because you say it is gets at the heart of the ad ignorantium argument.

It's all about evidence.

This year in English 9 I'm pretty much overwhelmed by the number of literary terms and vocabulary I have to teach, not to mention the numerous grammar concepts to introduce and/or reinforce.

But next year I really am going to have important fallacies studied--even if we just cover one a grading period. The post hoc ergo propter hoc will be one, and I think this ad ignorantium would make a very good second one. Two to go before next year!