Haha I'm pretty much in a similar position, but my teacher told us that the 3rd 'art' paragraph should include explanation on how Yeats uses the poetry art form to essentially resolve some of the tensions and conflicts seen in the poems.
Eg. How he touched on the resolution on the meaning of life in "Among School Children".
Seeing that you are not sure what this 'art' paragraph is, do you think it is important to include?
I'm assuming that your teacher has suggested this as a particular structure she or he thinks works for students, but in a HSC essay, the markers aren't actively searching for an "art" paragraph. I think this is purely a suggested structure, not at all a requirement for good marks. I, in fact, believe that you should be talking about the manipulation of form in every single paragraph. I really don't think I can endorse a dedicated paragraph for poetic form, with absence of form in the other paragraphs, because I don't think it's wisest. This is assuming that I understand what your teacher means by "art" of course!
If you were to talk about the poetic form for each - I'd be looking particularly at the poetic movements. The romantic period was departing prevalence in Yeats' earlier works, and it is considered in poems like When you are old and Wild Swans at Coole, that he's mourning the ending of this period. So he learns to adapt his style to modern features, whilst still treating his subject's with personal sensitivity. I'd also be talking about the deliberate features of poetry, like enjambment, refrains, repetition, stanzas, etc...All of these form features I think would fit into an "art" paragraph
Let me know if this sounds like it is or isn't in line with what your teacher is suggesting...I've certainly never heard the term before