But once you start to teach formal characteristics, with the terms, you need to state right from the bat not to expect too much regularity. I thought I had a tin ear for poetry, because all the teaching was emphasising the formal poetry. I stared at a line of ostensible iambic pentametre (or what have you) and I just didn't see it. I could have dealt with a couple of exceptions (nursery rhymes and songs do have those), but in much poetry irregularity goes beyond exceptions. It's a feature.
I'd certainly agree with that. If I can use my own stuff for example, my latest piece in the Crit forum is Missing Friends. The first two stanzas are:
Where I live there are no stars;
At most, the brightest planets peer out
through the city's choking haze.
I miss them, living here. I'd choose
a place where bluer days clear out
to studded velvet nights ablaze.
It's more-or-less iambic quadrameter, but the "less" is entirely intentional. For instance I dive right in with a single stressed syllable in the first foot. But then to give a more "lingering" feel, I end the second line of each triad with an unstressed syllable -the last foot of those lines is an amphibrach (daDAda). And no, I don't remember this stuff; I wrote it by the sound, and looked up the term for it just now.
This might not be a perfect example, in that I could read it so that the meter works perfectly but I'm not completely sure someone else could. But I think it's fair to say that Shakespeare does pretty much the same sort of thing: he just does it much better.