I feel that grades are more of a test of Work Ethic than actual mental competence.

I was ejected from the Seminar program in Junior High (highest tier of the GATE program, unless it's changed) for my lack of doing homework. I slept thru class, made high marks on tests/etc, but did no homework, and next to nothing for classwork.

As far as things go in the 'World After School', it IS your work ethic that matters. Grades are typically perceived to be something other than what they are, normally.