The teacher says "should be fine". It seems perfectly reasonable given that it wasn't said they "will be fine" to ask what the student does. Even given that however, IMO the student would still be within their rightness of mind to be sceptical of one person's opinion.
What exactly characterises "mental dullness; foolishness; senselessness" is itself a matter of perception, opinion; I think my contrary retort here shows that well enough — you think your example is good, I think it's bad, so who's correct? Rhetorical quesiton there, mind. My point is that by your (and many others') apparent logic every question would end up being called stupid by at least someone; so really, what becomes 'mental dullness'? If not the question, because now every question is stupid, then are their askers stupid? Which is tangential, ultimately not answering the question at hand. One could say stupid questions exist because stupid people do, but again, everyone ends up being called stupid by at least someone. Ultimately I believe you'll need a solid definition for 'stupid' based on measureable parameters, though I'm probably asking the impossible there...
I guess I sound a bit over the top about all of this, so don't feel obliged to waste your time answering if you feel it that way, but this is a discussion board after all. :D