Here are some alternatives you could use depending on the context:
* Instead of calling someone a "dunce", you could describe them as:
* "unintelligent"
* "slow"
* "not very bright"
* "having difficulty understanding"
* "struggling with the concept"
* If you are talking about a historical context:
* "In the past, children who didn't understand their lessons were sometimes made to wear a dunce cap."
Remember, using respectful language is always important.