Let’s break it down.
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There are countless reasons people do poorly in school–from primary to post-secondary–that don’t reflect on their intelligence. For example, I had high-school classmates who had to work evenings and weekends to help support their siblings.
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Even those who have the opportunity to do well but don’t–the ones I think you’re referring to–aren’t showing a lack of intelligence to do poorly. Judgement and intelligence are different things. I had classmates who could outthink me any day, but just didn’t bother trying. I still think they’re smarter than I am, even if my approach to school was more practical.
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Your schooling was very different from mine; I was being taught to write persuasive essays in grade 5. Sure, memorisation reflected in part of my grade, but some knowledge of the ideas is necessary to “put them together” at all.