Kinsei Private School (apparently a popular setting for Hyouta Fujiyama; there are a bunch of related manga coming out from June) is an all male school that's 90% gay or bi. This is a fantasy setup a la David Levithan novels rather than a political one a la Harvey Milk High School. Nanase, who is straight but cute and small, has to fend off crushes from many of his classmates, until his underclassman Heiji proposes a solution: They'll pretend to be going out. It will help Heiji, too, as there's a girl pursuing him he can't shake off. Heiji seizes the pretext to put his hands all over Nanase everywhere, and the revelation of his intent isn't far in coming.
What's fun about this, actually, is that Fujiyama gets the business of deception and misunderstanding out of the way pretty quickly, and proceeds to examine the shifts and changes in Heiji and Nanase's relationship. There's one cute up-ending of cliche in which an outsider's attempt to sow dissension among the two of them fails because they trust each other too much to assume the explanation for a compromising position is obvious.
The "Ordinary Crush" storyline takes up about two-thirds of the book; the rest is made up of rushed and confusing one-off filler stories, most loosely related to "Ordinary Crush" except for a story about a brusque scientist and the robot assistant who is, of course, more human than he is.