I've been looking for this everywhere but I forgot the tittle.
The story is about a student who fall in love with her teacher then One day she writes something on her chair/desk then someone replies, it turns out that the chair was a "time machine" and he was talking to a boy who also sits on the chair years ago.
Spoiler (mouse over to view)
The boy he was talking to was her teacher all along
I was also thinking that maybe this was a one-shot manga.
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