(Not a teacher)
In my opinion, "de-" is not acting as a prefix here, as the words "pose" and "depose" don't have opposite meanings, just different meanings, and have different historical roots.
It would be nice if "depose" (or de-pose) were to mean "stop posing" or "to stop from posing." My dictionary doesn't list this meaning, however. Indeed, "unpose" is not given as a verb either- the closest I can come is the adjective "unposed."
Student or Learner