IMO, no, not always. I've always been an 'average performer' in education, probably on the lazy side if anything, when I've put work in I've always done rather well, but one thing about most academic success is that (unless you're a researcher or PhD student) you're mainly regurgitating information you've learned from lectures, textbooks and independent research. To do well at that, you either need a bit of talent at it or just need to work hard.

But not everyone gets the opportunity to go far in their education, so they may leave school with no / few qualifications but it doesn't mean they're thick. My mum left school at 15 to work (this was in 1970 when it was common) and has worked in mainly manual work all her life then got married, but she's really good at maths, art and computers and I'm sure if she got the opportunity she could've did well.