Will students have studied poetry already and have a working definition of what a poem is? If so, you could measure how well students create a poem that fits that definition.
Here’s one I use, “Poetry is literature designed to convey a vivid and imaginative sense of experience, especially by the use of condensed language chosen for its sound and suggestive power as well as for its meaning and by the use of such literary devices as structured meter, natural cadences, rhyme and metaphor.” (Houghton-Mifflin College Dictionary, 1986)
If it’s just an in-class activity and there is no time to revise, I recommend giving students a COMPLETE or INCOMPLETE to indicate that they participated in the activity and finished it within the time constraints. I’d comment on what is particularly good, fresh, or interesting.
If, on the other hand, there has been time for revisions, consider setting up a grading rubric that addresses CONTENT, CORRECTNESS, and CREATIVITY.
For CONTENT: Does the poem include features you require?
* You could suggest that they write a found poem that summarizes the article or that takes an opposing view of the article.
* If about literature, write a poem that expresses a theme, describes a character or setting, reflects a key incident in the plot. See example of pantoums, narrative poems and ballads.
* You could ask them to title their poems in ways that suggest, but not tell, what they want readers to get out of reading it.
* You could ask them to suggest an idea that must be approved ahead of time to earn full credit.
For CORRECTNESS: Is the poem laid out the in lines that make sense? If punctuation is used, it is correct? If a particular pattern is required (limerick, haiku, structured rhythm or rhyme), how well does the found poem fit that structure? Does the poem seem complete or does it leave you hanging after multiple readings?
For CREATIVITY: What seems fresh, surprising, novel? You’ll know and so will your students who read the poems of their peers.
For more expansive descriptions of ways to introduce students to reading, writing and talking about poetry, see “Take T.I.M.E to Teach Poetry, Chapter 7 of my book TEACHING MIDDLE SCHOOL LANGUAGE ARTS: Incorporating Twenty-First Century Literacies.