My unlinkable "The Oxford Dictionary Of English Etymology" (dead tree edition) offers a bit more:
dunce disciple of Duns Scotus; dull pedant; dullard, blockhead xvi. orig. Duns, name of John Duns Scotus (died 1308), celebrated scholastic theologian, known as the Subtle Doctor, whose works were textbooks, and whose disciples, called Scotists, formed a predominant scholastic sect at universities until they were attacked by humanists and reformers; occurring first in contemptuous allusions in Tyndale's works in phr. Duns men, Dunces disciples, whence duns, dunce was evolved in the above senses.The dunce cap is no longer used in caricature, but occasionally, scholarly pedants use similar symbolism to derisively mock students of human nature.