Regardless of the "reading level" The Call of the Wild would provide lots of challenges for 12th graders-challenges in stretching, learning and discovering. As pohnpei said, this should not be a major concern with this novel. In all honesty, I completed a Frye Readability assessment on an Albert Camus novel, and it assessed at a 5th or 6th grade level.
I wouldn't really worry about it being too simplistic. What I would worry a bit about is the somewhat archaic English and all the dialect that's in the book. That might make it tough on people for whom English is already a foreign language.
For example, the word "mug" is used as a term for a person. The word "sou" is used to refer to money. We hear about an "express office." This is not a term used anymore. The term "cayuses" appears -- meaning a wild horse. More examples are plentiful.
So I would worry about that and the generally old style of London's writing.
As someone that teaches 12th graders, I have to say that I think Call of the Wild would not be too simplistic and that there are a number of interesting themes and ideas that can be addressed in more complex ways while still allowing the students to (hopefully) move easily and smoothly through the story for themselves gaining greater confidence in their English ability.
There is so much about the ideas of a meritocracy or "survival of the fittest" that led to so much of the really powerful forces that drove soceities all over towards things like eugenics, etc. at the turn of the century that I think there are tons of great things you can get out of it.