There's many story like that lmao, maybe this?
http://www.mangago.me/read-manga/bokura_no_melancholy/
If not, you can check the tag:
http://www.mangago.me/tag/teacher+x+former+student/
Thanks, but unfortunately it's not Bokura No Melancholy and it's not in the tag list either.
Now that I saw those mangas I agree there really are many same plots so I'll elaborate it more, as of what I have read about it.
The former student loved the teacher when he was in highschool. I think he started to notice the teacher because he was clumsy back in those days. I also think he decided to become a teacher because of that teacher. After he graduated(?), he applied at his high school to become close to the teacher. But after working for I don't really know how many days/weeks, he started to notice that the teacher was not really as clumsy as he thought he was, because 2 students told him that it was unusual for the teacher to be in daze. I think he confessed to the teacher and the teacher was thinking about the confession that's why the teacher was in daze.
That's what I read about it. I wasn't able to finish it that's why I'm finding it again. please heelp

I fogot the title of this yaoi manga so I need your help ╥﹏╥
Story: Sensei x Former-Student-turned-Colleague Relationship. As stated, it's a story about a teacher and his former student who, years after, became his colleague, aka a fellow teacher.
Pleeeaasw heeelp(づ╥﹏╥)づ