Modals

[twakkll33]

Hello,
We are always told by our teachers that we should use (have to) when we report decisions taken by people other than us for example:
1.A teacher to his student: " You should arrive on time"
1.The teacher says I have to be on time.

But what if I am reporting the exact words (reported speech)?
I mean can't I say:
1.The teacher says I must arrive on time.
Isn't the last sentence correct?

[Tdol]

I think they're different:
1 The teacher says I should be on time. (advice)
2 The teacher says I must be on time. (their order)
3 The teacher says I have to be on time. (order of the institution)
2 & 3 can be used with a similar meaning- it depends on how they say it as to where the authority comes from.