“Aren’t
I?” is used in informal language.
“Am
I not?” is grammatical. It is formal.
Incorrect:
I’m a student, aren’t I?
Correct:
I’m
a student, am I not?
“I
am a student, aren’t I?” is accepted correct in spoken and informal
written English.
“I
am a student, am I not?” is correct in a formal context.
“I
am a student, amn’t I?” is incorrect. However it is accepted correct in
places like Scotland.