That's because educational performance is hardly caused by intelligence, and it'd be a damn shame if it were. People should go to school to learn things, not to prove how smart they are. Performance in school scales with effort, and hopefully causes the intelligence, not the other way around. Perfectly bright, intelligent, capable people hardly scrape by high school.
I'm having major deja vu, I feel like I've seen this exact topic brought up in a very similar way here in Spam before. Hrm.
Anyway, yeah, pretty much what Hank said. There are different kinds of intelligence. Most people only seem to look at memorization and accumulated facts as intelligence whereas there are many other areas like
'common sense,' problem solving, lingual, physical, etc. that are all various kinds of intelligence.