Much of this is going to depend on how much of Hughes' work your students have read. This will determine how much of an introduction to the work you will need. For example, of you have already established some of Hughes' ideas in his work, the cultural context in which he writes, and some of the motivating factors behind his work, the introduction to the poem will be relatively easy because you have established the timbre of the poem and the configuration in which the poem is composed. If this introduction has not been established and this poem becomes your first foray into Hughes, this becomes more of a challenging element because you will need to establish much of the conditions in which Hughes writes in the poem. I think that a discussion point would be to discuss how the reality of dreams can both galvanize an individual into action and/ or help to wither the resolve of the individual for the farther individuals get from their dreams, the more pain is experienced.
If this is where you are, allow me to suggest a couple of techniques that might work with your students. One natural discussion point would be in the notion of dreams. This might be accomplished in reading another of Hughes' poems, "Montage of a Dream Deferred." After reading this, discuss what Hughes sees as the nature of dreams. Upon this discussion, it will make sense to transition into "Dream Variations." Another and more exploratory tactic could be to have students talk about their own dreams. This can take the form of large group discussions or even smaller discussions. Some basic points could be brought out in the discussions such as why are dreams important, how they provide relief from what is into what can be, or even where there is hurt in dreams? These questions could "prime the pump" in terms of establishing some prior knowledge that students could possess in terms of recognizing how to approach Hughes' poem.