According to the website, Perfect English Grammar the modal verbs of obligation
have to / don't have to: strong obligation (possibly from outside)
Children have to go to school. (maybe the law forces children to go to school) (sometimes 'have got to')
must / mustn't: strong obligation (possibly based on the speaker's opinion)
I must study today. (I force myself to study)
In natural native speaking, do you care about that difference?
However, if we want to use must in the past, then we have to use "had to". E.g., ‘I had to study yesterday’.
So, are "I must go to school" and "I have to go to school" interchangeable or non interchangeable?**