One of these days I really should put together an essay on this. Kurt has so many attributes that can be read as gifted, I think: from his gender presentation (gifted males are often gender undifferentiated) to his perfectionism and narcissism (I mean this descriptively, not pejoratively), and being a late bloomer physically (asynchronous development is common). It's not all about academic performance, though as you mention, he's very good at French (and I suspect well motivated there by his own ambitions).

Heck, one of the things I would add to any potential Glee drinking game is 'Kurt says a French word and other people are confused'.

As likeasouffle mentions, his studying hard at Dalton does show some enthusiasm for academics. Possibly because, for the first time, he's being challenged, since we have his comment to Mr. Schue shortly before he transfers to Dalton that he's "not challenged in the least" at McKinley. Which makes me suspect Kurt may well be a student who skates through on B's with little effort and isn't terribly engaged with most of what he studies at school.