As an NES, but not a teacher, I would have no problem with "hi!", unless it was in a formal situation.
It would be seen as a friendly gesture.
I use "good morning" when greeting my (Thai) mother-in-law in the morning, she responds with "mornin'" and honour is satisfied. The Thai greeting "sawatdi krap" sounds somewhat South African.
I reserve "good afternoon/evening" for when my wife's daughter surfaces from her room after a night out!
Student or Learner