Student or Learner
I read Michael Swan's Practical English Usage and don't understand the explanation of 'present progressive'.
Both structures (be going to and present progressive) can be used to insist that people do things or do not do things.
e.g. You're not wearing /going to wear that skirt to school.
Question: Is it a command or refusal?
I guess you are trying to learn English by 'rules', rather than by what the words mean.
Look at these sentences:
You're not wearing that skirt to school?
You're not wearing that skirt to school!
You're not going to wear that skirt to school?
You're not going to wear that skirt to school!
Have a go at telling us how you think the inclusion of 'going to' in the sentences alters the meaning, the sense of what is being said.