I would say that those were fair interpretations, and that yes, "be going to" can imply a planned action.
Other contexts might provide other interpretations, however. Thus
1. You're not going to wear that skirt to school?
could suggest mild surprise, if the girl isn't wearing the skirt in question; while if she is,
2. You're not going to wear that skirt to school!
could suggest outrage and disapproval.
Student or Learner