Can I use "mustn't" to reply the question ?
I know usually when we ask the question: May I smoke in this room ? ,the reply can be : No, you mustn't
If I use "Must "to start a question ,for example : Must I get to school before 9:00am ? usually the negative answer is : no ,you needn't and we don't say " no, you mustn't " .This is the grammar we learn in China .

But some Chinese students are arguing: if we are in some specific situation ,we can reply " no ,you mustn't " to the question " Must I..."
Here is the imagined situation:
Bob and Mary are a couple ,they were on Titanic ,Mary found the ship was about to sink, she can hardly hold the ship's rail cuz the ship was leaning , but Bob couldn't reach her , then Mary asked : Must I jump into the sea ? , How would Bob reply ?
of course he didn't want Mary to jump into the sea,he wanted her to hang on in there ,can Bob say: NO, YOU MUSTN'T

or he should say: NO,YOU CAN'T

Thank you for your helpOr when Bob and Mary asked the third person: Must we jump into the sea ? can the 3rd person say: NO,YOU MUSTN'T ?

Question

Can I use "mustn't" to reply the question ?
I know usually when we ask the question: May I smoke in this room ? ,the reply can be : No, you mustn't 
If I use "Must "to start a question ,for example : Must I get to school before 9:00am ?  usually the negative answer is : no ,you needn't  and  we don't say " no, you mustn't "  .This is the grammar we learn in China .

But some Chinese students are arguing:  if we are in some specific situation ,we can reply  " no ,you mustn't " to the question " Must I..."
Here is the imagined situation:
Bob and Mary are a couple ,they were on Titanic ,Mary found the ship was about to sink,  she can hardly hold the ship's rail  cuz the ship was leaning   , but Bob couldn't reach her , then Mary asked : Must I jump into the sea ? , How would Bob reply ? 
of course he didn't want Mary to jump into the sea,he wanted her to hang on in there ,can Bob say: NO, YOU MUSTN'T

or he should say: NO,YOU CAN'T

Thank you for your helpOr when Bob and Mary asked the third person: Must we jump into the sea ?   can the 3rd person say: NO,YOU MUSTN'T ?

Answer

The answers are right in both situations, because the meanings are different.

"You mustn't jump into the water (because you will drown)" = strong obligation to avoid doing something

"You don't need to get to school before 9 (because lessons don't start until 10)" = no obligation, complete freedom

The rules you and the others have learnt are the same.  They are not variable.  The important thing is to understand the exact meaning and context.

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