In high school we had to communicate with some teachers on off hours.
Once b/c my English teacher thought I'd cheated on a test (no, I did NOT cheat) so he called from his home phone to our home phone, at like 9pm. We were shocked he would call so late!
And once b/c I was being continually sexually harassed in my Spanish class and my loser Spanish teacher never did anything about it and my Dad was seeing red and ready to hurt someone, and the Assistant Principal called him from her cell or house phone late in the evening, returning his call, to try to help sort things out before my Dad went nuts.
....there were a handful of other times, too.
Oh, once a teacher drove all the way to my house (which was a big deal, I grew up 30 mins from school, out in the middle of nowehere land) to give me 3 or 4 hand-me-down dresses from her DD. She was really sweet.
I have no idea what the appropriate "etiquette" is for all these situations. I don't really care. I'm glad we had good relationships with all my teachers. And I would hope if my kids were ever in public school, we'd also have these kids of good relationships with their teachers.
I don't consider 7:30pm too late to call anyone. But I guess it would depend on how close I felt I was to that person.