In late junior high and high school, the computer lab becomes a place where boys who are often socially marginalized can prove their masculinity. These boys and sometimes even their teachers will ostracize or openly mock girls and minorities in computer science classes. Computer science teachers often give sports-related or mechanics-related examples or projects, rather than examples to which girls could relate. (In the book they have some particularly odious examples.) Computer science books will focus on technical detail rather than real-world metaphors or applications. Thus, many high-school girls see computer science as a "math elective," "supersmart and unemotional," and "a place for nerds" and not as a powerful field which affects many others.
Computer games are one of the primary ways these computer geek cliques "prove masculinity," explaining the prevalence of gory, destructive games. Boys also like fantasy or adventure games to prove their independence from parents and other authority. These games provide a safe, predictable, controllable surrogate for social interaction, which can be unpredictable and lead to intimacy and opportunities to be hurt or seen as weak. Many girls are bored or disgusted with gory games, games with a lousy plot, or games without connection to the real world, and they thus turn away from the junior-high and high-school computer culture. The people designing games are mostly male, and don't make games that cater to girls' interests. (At the same time, "Barbie games" that are "based on the crudest stereotype of what girls like" are also not enough!)
Teachers watch for and give special privileges to the classic "boy geeks," typically obsessed with the mechanics of computers, but don't watch for "girl geeks" who often have interests in computers that are not as mechanics-driven. Teacher influence makes a big difference: many of the girls who applied for CMU's computer science program out of high school had teachers encourage them to take computer science classes, usually in high school.