Let's get this quite straight:
REGARDLESS OF WHAT EVER THE OTHER SAYS, just to follow:
If the fact the statement refers to (well guess you couldn't really totally ignore) is true, you say yes.
If it is not true, you say no.
Simple?
If it is true, assuming that I'm stupid:
You are stupid, aren't you?
Yes I am. (Because I am)
You are not stupid, are you?
Yes I am. (Because I am)
Assuming that I am not stupid:
You are stupid, aren't you?
No I am not. (Because I am not)
You are not stupid, are you?
No I am not. (Because I am not)
You should pay attention, though:
As for "plain" statements, Chinese students always get confused:
I'm sure you are not stupid.
No. (Meaning I am not stupid, which in Chinese, would mean I am. )
I hope you sir can finally realize the situation is much more straight and simple in English than Chinese.