I'm looking for a word or phrase that implies stupidity in the context of academics in the same way the term "straight-A student" implies intelligence.
2 Answers
Merriam-Webster has the definition...
dunce - someone who is stupid or slow at learning things
People with the learning disability dyslexia were once simply dismissed as dunces.
The etymology, with focus on the specific term dunce cap as used in a schoolroom/academic context, is more fully covered by this Wikipedia article.
For a more "modern" term (often used facetiously), there's always academically challenged.
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In case it helps, if they are no longer in school and did not graduate, you can say "drop-out."– DanCommented Aug 28, 2015 at 15:30
dropout
but I wouldn't say he has a lack of academic intelligence.