I'm looking for a manga I read a while back.
I think it starts out with the boy being late for school and climbing over a wall (maybe). He sees one of the school teachers and, assuming that teacher is a woman, falls in love with him.

That teacher is however a man and doesn't take the kid's love declaration seriously. But even when the boy realises his mistake he still decides to pursue him.
I think also the teacher might already be involved with somebody else.

Can't remember much more than that...
There may have been a scene where the boy sees the teacher in bed with the other person he is involved with and although it isn't technically cheating the teacher feels guilty about it. (I could be mistaking it with another manga here though...)

Boku no Suki na Sensei

Yay that's it thanks very much ^_^