n. backward in book learning; child or other person dull or weak in intellect; dullard or dolt
It is very impolite to call others a dunce.
Sentence in Classic:
Stephen listened to them in some wonderment for Boland was the dunce and Nash the idler of the class.
A Portrait of the Artist as a Young Man By James Joyce Context
In a school carried on by sheer cruelty, whether it is presided over by a dunce or not, there is not likely to be much learnt.
David Copperfield By Charles Dickens Context