Actually sam teasing Dean about his education, that's not a new thing. Its been a kind of staple for Sam to poke fun at Dean's level of education since season one, although the first time I really noticed it was in Croatoan. And Dean himself remarked in season five about having a "GED and a giv'em hell attitude', so I had no problem with Sam teasing Dean about that sort of thing.

What I have a problem with is "showing" Dean as stupid and that's usually the fault of the writers. Dean's not educated. He's not booksmart. What he is is canny and street-smart and I love when the writers show that. He thinks fast in a fight ad creates clever solutions to problems while on the go. It's what makes him such a great tactician and the nominal leader during most hunts or just out in the field.

Also, it is the province of yunger brothers to tease their elders. It's tradition.