Renai Shikakuka Phenomenon

For some unknown reason, people who like someone romantically now grow horns on their head, which drop away if they lose their love for that person. These are the stories of a group of teenagers who attend school together, with their usual romantic issues complicated by their visible horns. The first story opens with a girl who can't seem to grow horns at the appropriate time, even though she really likes someone.

(Source: MangaHelpers)