I've always thought that that was a bad way of framing that advice. Of course there's such a thing as a stupid question! "But isn't two plus two five?": stupid question. What kids need to understand instead is that stupid questions are the ones that most greatly need to be asked! That's how you fix the stupid!
Asking the teacher after class is an acceptable face-saving alternative to speaking out in front of everybody, but in the long run it may not be necessary. I suspect I'm not the only one who remembers "that guy who always asked stupid questions in our engineering classes" but mostly what I remember about him a decade later is that he kept acing those classes too: it turned out that nobody understood all the material; he was just the only one who was really dedicated to fixing his misunderstandings before getting tested on them.
The thing that struck me about "stupid" questions was that my high school chemistry class had a student who kept asking questions that I didn't quite have the nerve to raise. The teacher was also very solid (he didn't ask us to memorize atomic weights because he thought it was a waste of time, and, while he wasn't flashy, he went through the material very efficiently), but I think it was the student who asked the questions contributed to extraordinary scores for the class on the subject-based SAT.
The highest possible score was 800. The class had about 28 students. 6 of them got 800s. I was one of the 2 who got 795.
(If I do anything wrong here, please tell me. I don't know what I'm doing and would benefit from being told what I've got wrong, if anything. I've never made a top-level post here before.)
So, it seems like most people here are really smart. And a lot of us, I'm betting, will have been identified as smart when we were children, and gotten complimented on it a lot. And it's pretty common for that to really mess you up, and then you don't end up reaching your full potential. Admittedly, maybe only people who've gotten past all that read Less Wrong. Maybe I'm the exception. But somehow I doubt that very much.
So here's the only thing I can think of to say if this is your situation: ask stupid questions.
Seriously, even if it shows that you have no clue what was just said. (Especially if it shows that. You don't want to continue not understanding.) You can optimize for being smart, you can optimize for seeming smart, but sometimes you need to pick which one to optimize for. It may make you uncomfortable to admit to not knowing something. It may make you feel like the people around you will stop thinking you're all-knowing. But if you don't know how to ask stupid questions, and you just keep pretending to understand, you'll fall behind and eventually be outed as being really, really stupid, instead of just pretty normal. Which sounds worse?
Here, let me demonstrate: so, what tags go on this post and how would I know?
So, anyone else know of any similar things to do, to get back to optimizing for being smart instead of for seeming smart?