It is a manga where the uke is a student and forced into a relationship by his teacher after having been caught with cigarettes. When he graduates the teacher gives the student his watch....they meet up later and restart their relationship.

Hmm my memory isn’t very good but I remember cigarettes in these. Maybe it’s one of them.
Evergreen days
Trap (Ootsuki miu)
Irome
goodbye Lilac
Ano Kado wo Magatta Tokoro

There was a post about smoking and student-teacher relationship so maybe you both are looking for the same manga?
https://www.mangaupdates.com/showtopic.php?tid=120026&page=1#post768665

Quote from z_goes_undercover

Trap (Ootsuki miu)

It was this one. Thanks a bunch!