For my son, who’s 15, not a Child of the South and in a different country:
Teachers: Sir or Miss (never Mrs) as a term of address with no name attached, or last name only if less formal (so Sir in the classrom and Kerwin or Mr. Kerwin outside the classroom.)
Stranger/Not Close Friend Adults: Mr or Mrs Lastname, but he’s the exception. I suspect this is a holdover from my childhood manners, I’ve never been addressed as Mrs by any of his friends, and adults here are shocked (although nicely so) by it. When he was smaller, it was Mr or Miss Firstname, but that had to go away because it was quaint in terms of manners. But still, I insist (and am known to thump ears) on the formality for strange adults, because it’s POLITE, damnit. Grr, offa my lawn!
Close Friend Adults: First name, only because I know them and I know it would make them uncomfortable to have otherwise.
He also opens doors and pulls out chairs. Made his little girlfriend all swoony.
I’m a Child of the South, so for me it was always (on pain of death) Mr or Miss Firstname. One close friend of my mom’s was Aunt Firstname.
The Miss. Firstname was true even of my great great elderly aunt’s elderly spinster friends.
Relatives are Aunt or Uncle Firstname, if adults, even cousins (my Grandaddy’s first cousin is Aunt Firstname, for example.)
Even teachers up to about second grade were Mr. or Miss. Firstname, but thereafter Mr. or Mrs. Lastname.
(And before you leave the house you get asked, “What are your manners?” and you answer: “Yessir, Nossir, Please and Thankyou, Mr and Miss and ThankyouverymuchIhadanicetime!” Or maybe that was me. I miss you, Aunt Wese and Grandma, I miss you so much.)