I don't see the difference

Hi Alan and Torsten! Will you be so kind to answer my question. I don’t see the difference in following expressions: I am to go to school… I have to go to school…I must go to school
Thank you

Well, I think the differences in meaning of those three expressions are rather small.
I would say ‘I am to go to school’ is more official way of saying ‘I have to go to school’ and ‘I must go to school’ means that you have decided for yourself you can’t but go to school.[YSaerTTEW443543]

TOEIC listening, photographs: A river view in Saint-Petersburg[YSaerTTEW443543]

Thank you Torsten