Okay, I've read this manga a long time ago so many things aren't really clear, I can recall some elements of the plot and details so the description will have spoilers!!! Here we go:

I remember it was a Student-Teacher Relationship, the male student confessed/proposed (not clear) to the teacher at the very beginning of the manga. The teacher had long hair but he had a short hair cut at some point of the story. At the end of the manga the teacher returns to the school and shows off the proposal ring to some students/friends (not clear).

My memory sucks so it's possible that some things are incorrect (perhaps it is not even a yaoi! [the teacher looked really feminine and wore a suit for classes] Hahahaha but better to be careful and submit it here...)
I will appreciate any help! 🙂

Found it! Boku no Suki na Sensei.
I opened and checked every manga under Student-Teacher Relationship tag, lots of work but worth it! Thnx anyways!