Traditional poetry is often tightly structured and sometimes requires a knowledge of poetic devices such as alliteration, assonance, and rhyme. Boys may find poetry rather feminine. Some students may have had experience writing poetry in L1. To deal with these concerns, I first explain about free verse. Poetry doesn't always have to be organized into iambic pentameter or rhyme and scan perfectly.
Motorbikes are as good as flowers.
There may still be a reluctance to get started. To get things moving, I have tried reading my poems to the class. I've also written a couple of verses in Japanese and read these out to amuse the students and let them know that grammatical accuracy is not the target. The procedure I've suggested involves getting the students to write poetry without first giving them any specific written examples. It has been easier for them to write their own poems than to have them interpret poetry written by someone else.