I’d say “visitors to the girl’s dormitory must wait in the hall”.
I think there’s no significant difference between “have to” and “must” in your sentence. Although I think “must” sounds more “demanding” and formal than “have to” and therefore it should be given preference over “have to” in such imperative sentences as yours.
That said, I don’t understand why your teacher didn’t pick up on your preposition choice. As far as I know “visitors to something” is much more common than “visitors of something”. Also, I believe the term is “girl’s dormitory” or “women’s dorm” and not “girl dormitory”.
Because Dear, must denotes something that is abslutely essential to do. here is not like that I think.
that’s why I said have to is more appropriate.
otherwise for more information we have to look for “must & have to” exercieses here.
as you are a teacher maybe you are right & I haven’t learnt the rules of english grammar that well.
if you have points, pls share.
Pooran