I don't understand why some people say that this development is surprising when the whole story has focused mostly on Eiji, Koharu and their relationship.
Like Koharu and Eiji have so much influence in each other through the whole manga, example: because of Koharu actions (kind of, is like a sequence) Eiji decided to stop avoiding his feelings and confessed how he felt to Izumi and Bessho, and because of Eiji influence Koharu stops the whole thing with her boyfriend and also stops pretending in front of Izumi and Bessho learns about her feelings for him.
And in more recent chapters they have become more conscious of the other, like Eiji noticing Koharu near the shoe lockers or when he called her when Sugi was near or whenever she's around Taichi. Koharu feelings are more ambiguous but there's last chapter when Eiji and Ikezawa were together and she decided to go with Taichi after all (jealousy maybe??).
So yeah, just because something is not spelled out doesn't mean is not happening.