Should we expect elementary art teachers to treat creativity and convention differently than middle school English teachers?

Recently a teacher candidate in a course I teach posted these images of student work from her elementary art classroom.

During a lesson on drawing vases, Jen showed her students how to create the lines that form the sides of the vase and then how to ornament the sides with lines and decorations in "smiley faces" that appear to curve around its contours and give the vase the effect of three dimensions. The images are by two different students from the teacher's elementary class. As you can see, one drew something that looks like a vase, while the other drew something that...doesn't.

In her commentary on our course wiki, the teacher wondered why the student who created the vase on the left hadn't followed the directions. When I read her post, my first reaction was, "Why should she have to follow the directions? Isn't what she drew beautiful (at least to her)? Especially at the elementary level, shouldn't we be encouraging creativity and self-expression over conforming to convention?"

Then I thought about my own experiences teaching English Language Arts. If I had assigned an essay to my students, and I had shown them the conventions I (or another audience) expected them to use in composing that essay, I too would be frustrated by a student who hadn't followed the directions.

In fact, I remember receiving an essay once on The Great Gatsby that began with a description of a glass bowl owned by the author's grandmother. When light struck it, this bowl refracted the light into different colors. Likewise, Alix argued, Gatsby was the prismatic central character around whom the other characters were arranged.

But the essay didn't use a single quote from the text--only examples and events--so I marked it down. I'm not sure I would do that now, but I'm guessing many teachers would have responded that way to such an essay.

On the other hand, we don't seem to hold other kinds of writing in English class to the same standards. For instance, many English teacher candidates tell me that one can't criticize or ask students to revise poetry.

But when they say this, I show them Robert Frost's first draft of "Design", and I share with them this poem by Jonathan, one of my former eighth graders:

There is a ripple
In the old fisherman's creek
The line is well cast,

The worm is squirming,
Fighting himself off the hook,
The nylon line pulls,

And the old rod bends.
It is a bite, a struggle
Against the fishy.

What a great poem! But what feedback would you give him about the last line?

So Jen's post about her students' vases had me thinking: Should our ideas about creativity and convention differ by discipline? By grade level? By mode or genre? What do you think?