You are not allowed to leave the examination hall during the first 30 minutes and the last 30 minutes of an examination.I could be really pedantic and say that this means that you can leave during the first 30 minutes or you can leave during the last 30 minutes just as long as you don't do both.
The traditional rule in Standard English is that "not X and Y" means you can't do both X and Y, though doing one of them and not the other is fine. In contrast, "not X or Y" means that you are not allowed to do X and you are also not allowed to do Y. (I have mentioned this before, in connection with "No shoes and slippers" here.) Presumably, the intended meaning of the announcement is that both X and Y are against the rules, so according to the traditional rule, it should be or rather than and.
But is it really true that the exam announcement is badly formed? I suspect that only a few linguists and other pedants like me might notice anything noteworthy about it; and virtually everyone else, both expatriates and Bruneians alike, will understand it perfectly well without batting an eyelid.
I previously argued that "not X and Y" to mean "neither X nor Y" is becoming the norm in World Englishes. This announcement is a good illustration of how this pattern can be used with no problem at all.