The Collaborative International Dictionary of English v.0.48:

Dunce \Dunce\, n. [From Joannes Duns Scotus, a schoolman called
   the Subtle Doctor, who died in 1308. Originally in the phrase
   "a Duns man". See Note below.]
   One backward in book learning; a child or other person dull
   or weak in intellect; a dullard; a dolt.
   [1913 Webster]

         I never knew this town without dunces of figure.
                                                  --Swift.
   [1913 Webster]

   Note: The schoolmen were often called, after their great
         leader Duns Scotus, Dunsmen or Duncemen. In the revival
         of learning they were violently opposed to classical
         studies; hence, the name of Dunce was applied with
         scorn and contempt to an opposer of learning, or to one
         slow at learning, a dullard.
         [1913 Webster]

WordNet (r) 3.0 (2006):

dunce
    n 1: a stupid person; these words are used to express a low
         opinion of someone's intelligence [syn: dunce,
         dunderhead, numskull, blockhead, bonehead,
         lunkhead, hammerhead, knucklehead, loggerhead,
         muttonhead, shithead, dumbass, fuckhead]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "dunce":
   Boeotian, block, blockhead, bonehead, boob, booby, bufflehead,
   cabbagehead, chowderhead, chucklehead, chump, clod, clodpate,
   clodpoll, cluck, dabbler, dilettante, dimwit, dodo, dolt, dolthead,
   donkey, dope, drip, duffer, dullard, dullhead, dumb, dumb cluck,
   dumbbell, dummy, fool, gowk, greenhorn, greeny, ignoramus,
   illiterate, illiterati, jobbernowl, know-nothing, lackwit,
   lamebrain, lightweight, looby, loon, lowbrow, middlebrow, niais,
   nincompoop, ninny, ninnyhammer, nitwit, no scholar, noddy,
   puddinghead, put, stupid, tenderfoot, thickwit, unintelligentsia,
   witling